False bottom and silicon sealant questions..

I was thinking of building a false bottom out of a food grade plastic lid that I got off a giant olive bucket we have at work.  I know it's food grade safe, but does anyone have any idea if there would be any problem with the lid at mash/sparge temps?  Maybe melting or not being food grade safe at that temp?

Second question, I built myself a mash tun, in the typical cooler style with a ball valve out, and used rubber washers for spacing.  I then sealed it with aquarium safe silicon, but noticed that the silicon turned green.  It didn't do this on the other one I built, which used silicon o-rings instead of the rubber washer.  Thinking maybe it leached something out of the rubber (or ate it a little) when it was drying, any ideas here?

Thanks ya'll.

The $1 grey replacement bung for a 5 liter minikeg fits most cooler holes(once you remove the plastic spigot) really well.  Mine popped in and doesn't leak.