I recently did this mod a month, or so, ago. Had to purchase two different units until I got one with the right diameter. The good news is Autozone, O'Reilly, NAPA, AdvancedAuto all carry the same two units. Get the smaller diameter one with the flip up lid. I think it was 8 or 12 bucks.
Things to note:
1. You'll have to bend in the two tabs on the side of it for it to work. Will make sense once you buy it.
2. The electrical connectors on the back make contact with the mount point on the firewall. You'll need to bend the positive terminal down 45-60 degrees and wrap electrical tape on the mount point (to be extra safe). If contact is make the fuse will pop. Both units from the parts store are longer than the RadioShack unit and have this issue.
2006 Lotus Exige: Avery Matte Graphite wrap, partial built motor (9:1 Mahle pistons, Ferrea Competition valves, ACT Clutch, ARP head studs), REV400, Nitron SAs, BOE QS exhaust, Ultradisc rotors, Moroso Oil pan, Circuitworks oil pump, CF Stuff (Cup Roof, ReVerie diffusor, side scoops, APR air dam, craftsquare mirrors), Worksbell quick release, Odyssey PC625, Morimoto Xenon HID, BOE Rear Oil Cooler, Difflow Aeroplates, BOE Swirl Pot, DaviesCraig EWP, Reversed Heater Matrix