I'm trying to find flanges for 1/2" PVC, so that I can bolt the construction to a table. However, all the ones I can find are for Schedule 80, which is too much for my needs. Do sched 40 flanges not exist, and is there something else I can use instead?

I went to the mother of all parts stores to try and find your answer, McMaster-Carr. It looks like they only offer that are schedule 40 are for BIG drainage systems.

You can use schedule 40 pipe with schedule 80 connectors, as long as you aren't ever running water or air through the pipe.

Have you considered using metal pipe flanges? You could get a NPT PVC adapter for the 1/2" PVC and make a mechanical connection to the metal flange.

Holler if you need more suggestions!