Lipos do vary in dimensions. Restricting the size of the battery compartment can limit what batteries would/could fit in the future. Also a little extra room can allow someone the ability to move the battery around to adjust the balance of the car.

As far as taping batteries, it is common on pan oval cars and many carbon fiber on-road cars to tape the batteries to the chassis. An easy solution for vertical movement on a chassis like the new M-05 is to use a foam spacer between the top of the battery and the chassis.

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For parking lot bashing, velcro is a decent option.