I've seen plexiglass spacers on some of the enclosures (under the lid)
Does somebody make these, or are they a custom made thing?
Do you mean the clear plastic things? I believe those are referred to as "light plates"
Yep, that's what I'm looking for
Don't they sell them over at DIY stompboxes?
Smallbear sells them, last I checked.
You can of course "roll your own". If you do, try use lexan instead of plexiglass. Its easier to sand and less brittle.
FYI: you will need longer screws to get the lid on your enclosure. Some sites provide them, some don't. Smallbear has several light plate sizes for different standard enclosures.
~Steve.