I bought one of those water tight cases haven't used it yet. I got a Tupperware thing and plopped it in there under my seat. About the only water my car sees is when I wash it so not too worried about it.
Almost all race guys move there immobilizer up under the dash. Granted they only have to extend a few wires. I've always thought about extending it up against the firewall under the carpet. It's not that far and could be tucked up inside the hollowed out carpet.