I had a cabinet with lots of shelves, it was perfect for my 1/10th RC cars but it was just a little to short and sometimes the cars would roll back and fall out so designed this simple stopper to put a little bit of an angle to keep the cars in. Can also be used if the car is a little too long as doesnt need to be flush.

Takes 3mm countersunk screws, mine were 12mm long but that can depend on what size wood you're screwing into. You could also use sticky pads if needed. Theres slots for the screws to allow some adjustment.