Looking for a kit like FM's undertrunk kit for the NC, but for an NA/NB. Just want more trunk space. Under the soft top would work fine as well. Anyone sell something like this as a kit?

 

But North American NA/NB Miatas need all the right rear weight they can get. I've not heard of a battery relocation kit for them. Usually performance-oriented people are welding steel into that area to get proper weight distribution.

 

Moving it to the right rear underside wouldn't change weight distribution, just would lower CoG. Moving it right rear under the soft top would keep the distrobution about the same, but decrease the MoI, etc.

It's not like I'm moving it to the driver's side or up front.

 

Relocation under the current location on a NB would mean scraping the ground.

The trunk area are very different between NA and NB, if you bother to look.

 

The car also has two seats, while we're stating the obvious.

 

If you need to retain a full-size battery, it's already in the best place. If you can downsize to a LiFePO4 motorcycle battery, you can probably tuck it in next to the gas tank or even behind the passenger seat.

 

Moving it under the trunk means moving it closer to the muffler and its associated heat. Unless you've done some kind of fancy exhaust that moves the muffler or makes it smaller.