RFI packs are also flat bottomed. We run SMC, ThunderPower, RFI and all are respectable. I've had others that I wasn't nearly as happy with. Some fit better like SMC 4100, 4200 and RFI packs are thin and fit pretty much everything from TC to 4wd off-road. The SMC 5200's are awesome but too thick so they're a really tight fit on most cars.