This is a laser cut robot design which uses 3 standard servos (I used pretty much the cheapest I could find online). Movement is done via a linear actuator-driven lifter which allows the servo half-wheels to reset before rotating forwards again.
It has a mount for a BBC Micro:bit and a DFRobot Micro:mate expansion board, and uses a 2000MAh phonebank charger to power it.
It's a bit on the fiddly side to assemble for the innards.
The design assumes a 3mm material for the laser cutter - I use MDF. Tolerances are quite tight so as to assemble without the use of glue.