Bleebs ... it depends on the CG. Stick the pack into the fuel tank compartment and check your CG. That will give you an idea how to secure it. If it needs to be really in the compartment then:

i. cut an access hatch from above
ii. build a platform for the batt
iii. stick velcro to the platform (hooked side) and the base of battery needs one too (fluffy side)
iv. use a strap for double security

If it does not need to be pushed so far up front, like my GP Big Stick, then build the same platform and access via the electronics compartment. In the case of my WM Sky Raider Mach II, it needs to be right in the compartment, I was kinda lazy so I stuffed it with a lot of foam to keep it in place. Not the wisest of methods but it worked.