I made some round planetoids using polystyrene balls. Mine came free from work (bin diving) but you can buy them in various sizes from craft shops. I poked a hole in them with a skewer and glued a piece of brass tube into the hole for a permanent mount which could use a normal starship base.
It has just occurred to me that you could add the craters before the cover by poking it with something hot or sculpting it with a pyrogravue.
I tried coating them with several different materials:
PVA and sand. Easy but a bit boring.
Filler. More work but you can get a nice effect with a bit of work.
Acrylic texture gel. Good, easier to work than filler I think.
Liquid cement. It eats the polystyrene in interesting ways. Do not try this inside.
A very long time ago I mounted ping pong balls on starship bases as bogie markers for GDW's Star Cruiser game and just sprayed them black. They are still is use even in Star Cruiser is long gone. I mention them because a ping pong ball might be a good basis for making spherical planetoids.
For irregularly shaped planetoids you have:
Rocks and barbeque lava rock (pumice perhaps). Potentially good looking but heavy (if only relatively so for pumice) and hard to mount.
Bark. The lumps you get for gardening in a big, big bag.
Cork bark. You can buy little bags of this.
Polystyrene chunks. Just shape them by pinching bits off with your fingers or go postal on them with a heat gun or pyrogravue but as ever be wary of the fumes.
