i had a setup quite similar to yours. it was a 3s flightmax pack stood vertically in my rustler tray. I used a strip of 1 inch wide sheet metal bent into the shape of the pack, and put on those 2 posts at the front. Now i went to lowes, spent like 7 bucks and got 10 feet of velcro. I just drilled a 1/8th inch hole in the chassis and velcro. then put a washer on the velcro side, and used a m3 nut and bolt to hold it all in, alot like Brian's. Either solution works fine. I just prefer the velcro because i always lose body clips. The velcro comes just like this ->
http://www.rcplanet.com/ProductDetai...30-247&click=3
Just in a roll, with no adhesive on the other side. This way you can cut right to length. With an inch of overlap, itll never come out