I've found PVC caps to work well for buckets/enclosures for some applications, 1/2" I think, the smallest size.
I generally use hot glue or Shoe Goo. A great way to apply Shoe Goo is, not sure what it's called but the pharmacist gives them out, baby medicine shooter or something. Kinda like a syringe but no needle. Pull the plunger out and squeeze the Shoe Goo in. Works great, I'm actually only getting it where I want it now!
Also, here's a set of tail lights I made with some red plexi and styrene. The reverse lens I drilled out and filed square and used a piece of lexan.
