For these purposes I suggest you grab a piece of pink foam from Home Depot: Foamular by Dow Corning
It comes in various sizes, but since I live in a small apartment I always choose 2'x 2' squares, which run about $6. You can cut it up into 8" blanks with a box knife. I holds detail very well. You can glue it to itself with Gorilla glue, (do NOT use super glue!). It cuts nearly silently - certainly quiet enough that I can run jobs at night when the kids (and wife) are asleep. And best of all, you can run through this stuff pretty much as fast as you like without endangering your mills, which will reduce testing time significantly and save wear on your cutters.
I use it routinely with a .125" end mill at about 7500rpm, 85 IPM, 37 plunge, 90% stepover, 100% step down, and I'm pretty sure it could go faster. Tip: give everything a quick wipe with a dryer sheet to make the styrofoam chips easier to clean up.
As has been noted before on these forums, Loews also sells a version of this which is blue, and does not work as well for our purposes.
Here's a picture of the bottom part of a passive speaker for an iphone. It's been floating around in a drawer for a few months, so it's a bit banged up.
Here's a closeup. This is the finish you get when you run flat-out. It's not lovely, but it's perfect for proof of concept stuff and testing.
Here's a toy free weight that I'm making for my kids. Ran it more conservatively - still faster than the MeshCAM defaults for Wax I think, so the finish is still a bit fuzzy, but this stuff sands very easily with 320 or higher. It is paintable with latex, and if you prime it with some Killz2 you can spray paint it with Plastidip so it will look like rubber. That's the plan for the weights.
Hope that helps!