Seeing how your doing a lot of custom work... there are a pile of solutions here:
http://www.jegs.com/c/Batteries_Battery ... 2/10002/-1
I will be using one of the options here I just haven't decided yet. In my spider the battery was originally in the engine bay. I have filled in the hole where the rusty battery tray used to be and am moving the battery to the trunk.
A lot of the stuff I use comes directly from the hotrodding suppliers or a junkyard from any car in the yard not model specific... Unless it is a body panel, control arm. Steering Arm or tie-rod and that has to come from a FIAT supplier for sure and I have to modify most those parts in order to use them on my car as well...
Rob