I would start by researching how to correctly wire the gauges and make them work first, then invest time into building the enclosure. First thing to do is to get a wiring diagram for the bike and stare at it until you comprehend what's going on. This took considerable effort for me, and unfamiliar diagrams made by different publishers still throw me for a loop at first. Stick with it.
Functionally, You need to know what type of signal the existing fuel and speed gauges use. Most modern vehicles use electronically operated gauges instead of old-fashioned ones that use rotating cables to assess information about wheel or engine speed.
The aftermarket gauges you buy will probably have information and strategies for reading or conditioning a variety of signals. You should look at the sales info for the specific gauges you want to use to figure out what type of input is required.
It would then be a good point of research to find out what signals are currently used by the gauges on your bike. You can do this by tracing wiring diagrams or taking apart the gauge cluster and measuring the voltage on the wires with a scope. You're looking for a frequency for speed and a variable resistance to ground for the fuel level.
The other indicators, like oil and temp, are just discrete on-off signals. If you want to add gauges for these you'll also need to add signal sending units, which will measure these variables on the engine and transmit electrical data about them (usually a variable voltage, controlled by resistance).
The tachometer will require tapping into the ignition coil's primary circuit. This is along the same lines as the speed signal; you need a signal which will pulse at a variable rate to indicate rotational speed. The tachometer you buy will have information on this as well; some are designed for low voltage signals from an engine crank sensor, and some are designed for the relatively higher ignition coil primary.
To do this work you're going to need, at minimum, wiring diagrams for the bike, detailed installation instructions for the specific gauges you're using, a good volt meter (preferably a scope, look for a used snap-on vantage or something similar on ebay, very cool tool)
Once you get that all sorted out and the gauges are reading correctly, you get to start building your custom enclosure. Maybe you want to use fiberglass or sill-brazed large diameter copper tubing! Just keep the backs of the gauges dry... or do you need to keep them totally enclosed? Sure sounds like a project.
