There may be spoilers ahead.

Farming those claptrap parts is by far the most repetitive and time consuming stuff to do on the road to 100% in Borderlands.
Please be warned. If u consider to use this guide (and walk from spot to spot) it may take a few hours to complete any achievement related to the Claptrap part farming. More information on my approach further down!

The Alternative

If you haven't killed MIVAC (the final boss of the DLC) yet - I really recommend you to fire up google and search for 'Borderlands 1 Claptrap parts farming'. You will find many guides for an afk-farming approach which saves you a lot of nerves to complete this. You can do it in your second playthrough as well if you already finished the DLC.
There are theories that all your progress/counters for the achievements stay with you as you start your second playthrough but thats not confirmed in any way. If you know something about it or tried it yourself, let me know in the comments!

Whats the goal of this guide now?

So - since I do not want to start a second playthrough, hate to do afk farming and already killed MIVAC without thinking about completing the achivements - I now have to find me a way to complete it by grabbing whats left behind. There are a few spots where those little rebels spawn. I want to collect those spots I found in this guide to give u a helping hand so that you won't have to search as long.

If you want a deeper dive into whats ahead of you - I found this page summarizing some hard facts about the collection of Claptrap parts, including an approximate table of dropping chances.
Be warned, having a look on your chances may lead to a serious sanity loss.

Weblink here [borderlands.fandom.com]

If you are interested, I started off doing this at the end of the DLC. In conclusion: some clappy parts were already in my pocket and my character was already overleveled for the claptraps spawning.

I did it with Lilith. Her Phasewalk will save you a good amount of time walking through the levels as there is no fast travel system in the DLC.
At the time my character was at level 41, one shotting all the lvl 36 claptraps. I highly recommend you to gear up a little if you need more then one shot for those little guys - as it will save you time and you are able to ignore all other enemies in the area.

... now pile up all the patience u have and get started to farm!

• Always keep in mind that every spot needs a certain time to respawn your enemies.
  
• Start circling the spots to get the most out of your time.
  
• Keep track of the rare drops you got - such as Pizza slices, oil cans or panties etc. (May use the Steam notebook @'shift+tab' in game)
  
• Do NOT alt+f4 the game at any point

Recommended walk cycle

• Just circle between spot 3 and 2.
  
• If you are fast you may have to wait a few seconds for enemies to respawn but the timing fits nearly seamless.
  
• Skip spot 1, it adds a ton of walking and there are not too many claptraps to farm

Alternatively you can just camp at spot 3 if all the walking drives you insane. You can close the guide now if you choose to do it like that. Just find the spot and wait for wave after wave.

In case you don't remember the layout of the main DLC map, I marked you the entrances to the spots here. 1 and 2 are connected by the rails. For 3 you will have to get up the tower behind Tannis office again.

Just walk straight up the huge Bridge from the entrance of the level to the point where you come down at the end. You will find some claptraps on there. After that you can leave again.

There are not too many claptraps, so consider skipping this one scince it adds a lot of walking.

(Note: There will be a gate at the bridge that gets unlocked through the story of the DLC, hope u opened it up with the button, otherwise you will have to take a walk through the whole map again)

You will basically do half a round through the city, including the Claptrap factory. Start by crossing the bridge into the city, leading to (1) Where some claptraps are randomly spawning on roof tops and gates. After that roam to the bandit bridge (2) Claptraps will start spawning and the bandits there will help you to clear them out. From there it is not far to get to another Pipe-like spawner in the corner of the map (3). Dont miss it out, there will be a good amount of claptraps plus vending machines to instantly sell your loot if you want to pick it up.
Last stop is the factory (4). Clear the main construction hall and then move to the three cooling facilities you blew up during the story.

(Update: There is another spot when leaving the Factory through the main entrance going to the right. It is up a ramp. A few more claptraps are spawning there)

After that you can leave.

Starting from the entrance of the map, stay on the left path. There will be a Hyperion-clap camp in the middle of the spot. All around it are huge Pipes where the small rebels come out from.
You can actually camp this spot to wait for them to respawn, but it will take some time. If you want to actively use this time, gap out to see the other spots.

Thanks for reading my guide! You must really be wanting those achievements just like I do!
Tell me your experience, leave some feedback and may share the time this took you to complete down in the comments!

Keep on achieving traveller!

My Stats @7,5 hours (time to finish all achievements):

• Fishbags 55
  
• Panties 35
  
• Oil cans 27
  
• Pizza 18
  
• Bobbleheads 15

Circled all 3 spots for the first 3 hours. Could have absolutely saved me some time by just circling 2 and 3 from the beginning.

Unlocked related achievements: