Well looks like this has never been done? i will do it and post a thread sticky about it then. Be a first cool.
I haven't done it, but I if were to do it you already got good advise.
Measure a metal piece to use to mount the gauge, 3/4" wide maybe 2-3" long
Measure the spacing needed and drill and tap
Also drill small hole on each end
Tie wire or string on each end
Tie loop in one end of the string
Made a wire fish making a hook on one end and make wire into an arc
Start the string and bracket thru the cowl
Take wire and fish the string out of the cowl thru the opening needed
Either bolt another piece to the top or the actual gauge bracket to it.
As far as hooking the gauge up need to determine the best way to get the wire/line to the gauge and hook up before mounting the gauge if it needs to go thru the cowl/firewall
Really not that hard to do.
I don't fish but have been fishing a lot.