I bought a Antec NKS 1300 very cheap at Fry's ($65 minus $40 mail-in rebate). I figured, why not? Well, it's the smallest mATX case you can buy, but it is still too tall.
I run a 4 gauge Moster cable from the battery to the trunk.
This is the solution I found, suspended under the rear deck. I removed the stock AC power supply, put the DC-DC DSATX there, and also mounted a 120mm 3 speed Antec fan.
The advantage is that I can carry my water bottles in there, since I live in earthquake domain, California, so I always have to be prepared.
It is rock stable as it is, and it leaves plenty of room for a stroller and a golf club bag. It also leaves room for the spare and for a future subwoofer.
All in all, it works. It's not perfect, but I'm happy, my first boot was good, it works!!!