Hey! I just saw this thread, I guess I'm a little late but I want to throw in a nanny word--
Phenolic plastic floral foam is carcinogenic, and florists are urged to only handle it wet to keep from breathing the dust. Sanding it dry like that would have gotten a look of horror from my florist mom, for what it's worth.
If you're gonna make more foam stuff, far better to use xps styro from somewhere like aircraft spruce, if you are using epoxy resin. If you are using polyesther resin, urethane foam.
Ok, captain safety signing off. Looks good btw--