I suggest cutting the back off of the gauge cluster, then taking some acrylic and building a rectangle box to fit the gauges in. add what gauges you need, using the remnant of the stock speedo cluster as your mounting system, and then simply run wires to the spots that the stock gauge cluster had wires run to (i.e. use wires instead of the "motherboard" thing on the back of the stock cluster.
then it would be waterproof, and you can add what you want if you leave a few extra spots in there. plus lighting it would be easy, and if you put a screw-on faceplate in there with a rubber seal, it will remain waterproof (provided you waterproof your electrical connections) but still maintainable.
do you follow what I mean?