Buy 3 of these "Swivel Caster - Gray Soft Rubber Wheel" - Model 3 = $30
Two pieces of whatever wood = $??
A few screws = $0.??
1. Screw 2 pieces of wood together perpendicularly.
2. Drill holes at the end of the piece of wood and mount casters.
3. Put a screw through the piece of wood above the wheels as shown if you want some wheel angle control.
4. Put your tripod on it. Secure it with cable tie. Done!
Dolly shot sample 01
Dolly shot sample 02
Link to HV20.com thread:
DIY Motorized Mini-Dolly test