This video just demonstrates a Simple, low cost and easy to make foot adaption pad.
Parts used:
2x servo horns (standard - heavy duty)
4x Ext springs (5.6x25.4mm)
1x Connector Hub (optional)
1x Pololu Ball Caster with 3/4" Plastic Ball (oiled)
with smaller springs and a flatter surface on the bottom horn i think it would work quite well.