There are various ways of mounting the battery within the frame.
Lionel appears to have held his in place with leather straps and buckles:
Gary opted for Velcro instead:
Roberto constructed a metal box to house his battery and controller:
Pavel opted for a major frame modification to fit this battery in:
There are many options, but you will need to choose one that suits your particular battery and frame.
You will also need to take into account your practical skills and available equipment etc. too.
Alan