I usually mix about a cup at a time. This piece took at least 5 batches. I use Bondo Phase 2, which is intended for autobody fiberglass repairs. It is a grey/green coulur, but can be tinted with powder dyes. I have recently found a Canadian supplier for Alumilite water clear, so may get a jug of that.
I previously did one like this and mixed about 4 cups at once, and the thing started to smoke. It was way too hot to touch, and cracked a lot while cooling/curing. It is still laying in the corner of the shop, waiting for inspiration or disposal.