Tree Holders and Such.

You could use wooden spring type clothes pins. The jaws can be modified to what ever you want and then can be screwed to a block of wood, etc and set inside the spray booth when you need it. The clothes pins will keep those trees from flying unless of course you have a tornado

Here is what I use,

Go to radio shack and buy a pack of large "A" clamps, use a 1/4" round wooden dowel and make them about a 12" long. Put one end of the dowel into the rear of the clamp and using pliers pinch the end of the clamp to hold the dowel in place, I made mine up about 10 years ago they are still going strong. Here is the best part, put up a small piece of peg board on the wall with a 2" spacer so that the peg board stands away from the wall and you now have an instant parts holder. Check out pictures of my Work/Model bulding shop at the following link.

http://www.naplak.com/modeling/Modelers/workspaces.htm

Big Jake

Some of the upper scale electronics shops carry what is more or less a soldiering helper. Its a heavy based holder with two "roach clips" on independent arms that each have 2 elbows . I like that it has two, one some times isnt enough and the spruce will flop around a bit.
Some models even carry a magnifying glass for "detail " work.