A very simple solution is COMPLETE industrial off the shelf designs.
Buy a.... micro car.... at a cheap discount / dollar toy store. Read the vehicle BATTERY requirements........Done Pick a ....AA....powered vehicle. It could even be Barbie.
There is VERY LITTLE new electronic breakthrus to be made any more.
Knowing how to search for adaptable parts is what made me a sucessfull retired redesigner.
Companies LOVE people who find existing parts to do a project.