Onmi wheel with O-rings for better traction. Print out two of the OmniWheel and 16 of either the smallwheel or smallwheelhalf stl. SmallWheel will print out one of the small wheels using support while smallwheelhalf will print out two halves of the wheel that you can glue together. Then use #10 O-Rings on the small wheels (16 required per wheel). After assembly the dimensions are 8cm diameter and 2cm deep (including o-rings).