In the end I went with Libbybapa's final suggestion, I think that is the nicest way

...or use a piston platform with no connections and autodrop disabled.

You could also just recess that square 1/4 or use a 1/4 block on top of it and use one of my fancy offset portal magnets. :) Then the player would fall off the ledge before being able to make the portal shot.

That would require a standing fizzler. I hope that won't cause any z fighting :|

Just moving the fizzler so that it aligns with the glass Could be the best Option for the moment

Fixing that sort of thing will be another use for my 1/4 high glass item. :-)

Yes, of course I solved it the right way first ;)

Oh yeah, I tried the same. Totally legit.

Ha, I saw that before and forgot to mention it. I did not use it as it is obviously not intended. :)