It is a fun project to build a simple robot but, often, the hardest part these days is creating the mechanical base. [Concrete Dog] has a new open source design for stoRPer that uses a PC board as the base. The board has a Raspberry Pi Pico and motor drivers. The modular design allows you to add to it easily and use custom wheels. The video below shows some treaded wheels and some mechanum wheels with gears.
There are mounting holes for sensors and also a way to put another deck above to hold other circuits, power, or whatever you like. There’s lots you could do with this as a starting point.
