The density of pretty much all thermoplastics is very similar, it's what they're filled with that makes them heavier or lighter. Polycarbonate sheet can be quite dense, because they often fill it with glass to improve clarity and scuff resistance.
Don't know why everyone thinks they have to cast their own material for making end caps, I think it must be because the commercial units over in the U.S have cast endcaps, but they're made that way to enable simpler batch production. For one off's it's much easier and better to use pre-cast or extruded sheet stock, and machine them. Commercial modules in the UK e.g. OTW, Sheerline, A1 downunder all uses machined endcaps turned from acrylic.
As you have a lathe, this will be a cinch.
The lightest plastics tend to be the polyethylene type polymers. These actually float in water (just) and machine easily as the plastic is soft, but are difficult to bond to as they are naturally oily/waxy and they don't hold a tight tolerance very well.
The nicest plastic to machine is acetal/delrin. Is a very accurate plastic that doesn't tend to 'grow' as you machine it, but is also tricky to bond, and is quite expensive.
In a nutshell you can use any plastic to make your caps, just make sure they're reasonably thick to resist flexing, a minimum of 6mm (1/4") for up 2-3" tubes, and 8mm (3/8") or more for bigger cylinders.
Don't use bondo, it's probably the worst material you could use, having little to no tensile strength.