how about this for the top.
to maximise the area used to polish (and so speading up the polishing time).
how about this.
use 3 pieces of 1" plywooth , so u the total thickness is bigger then the size of a poolball. then cut the plate round = the innerdiameter of the bucket.
with a ***** (dont know the word) u cut the round holes in the wood.
put carpet around the inside of the freshly cut out tubes from the plywood.
put the balls in the wholes and turn the machine on.
something like this.
ps maybe the balls will spin less quick as there is more friction :s
no idea really. at least your balls will never tough eachother, its a easy spacing system to make..
what u think will work best?
u could even add a round top with carpet that once the balls put in place, u cover it with.
like that u have polishing effects beneath, above and 360? around.
easy to make , stable as u can use as much screws as u want to fix the circel.
my 0.2c. ill be making one this week

sounds like a fun and cheap project.