If you'd like to give your hand the freedom to wander, these next steps will show you how to add a simple axle and ratchet mechanism to the hand.
Making an Axle
Using a metal rod (or bamboo skewer) as an axle, we can make a rolling base for our hand.
Poke or drill a hole through the base of the cardboard tube.
Push a small straw through this hole.
Insert the axle through the straw.
Press the other wheel onto the end of axle.
Making a Ratchet
Using the tips of some bamboo skewers, we can make a ratchet that will prevent the wheels from rolling backwards.
This will ensure that the hand only crawls forward, not backward.
Use hot glue to brace these pins against the inner lip of the wheel so that it catches the notches as they rotate.
Wrap It Up
Tape the battery to the end of the wrist. Make sure the servo arm still has enough clearance to move forward/backward!
Use a square of double sided tape to stick the Circuit Playground Express the to back of battery.
You may find your hand moves better on certain surfaces, depending on how slippery they are.
If you'd like to give your hand more grip, adding a thin layer of glue to the wheels and a dab of glue to the tips of the fingers helps.
The battery that powers this hand should last a long while. If you need to recharge it, look at this guide to learn how to do it safely, and all about how LiPoly batteries work!