Heres an easy, inexpensive, and functional caster, made with two clothespins.
Step 1: The materials
First you need two wooden clothespins.
Step 2: Disassembling the pins
Take apart the two clothespins, but keep the springs. you can use them for replacements for the mini peg gun: http://www.instructables.com/id/DIY-Peg-Mini-Gun/.
Now that you have the two pins apart, time to move to step 3.
Step 3: Assembly of coaster
this step is simple, just look at the pictures. to put them together, you need to put the small end of the clothespins in the slot on the backside of each one.