Nipper, the only enclosure I've ever had was on a different boat and I paid someone to make and install it. They put in the snaps etc. I really cannot help you in that department.. I'd probably make it so the two corners were 2 inches apart and then sit there wondering what I did wrong.
As to tools and your other questions... You can buy a nice cheap set of 4" digital calipers for $9.99 on sale at Harbor Freight.
http://www.harborfreightusa.com/usa/tabviewer/startBrowseBook.do?bookid=275&preview=&type=RET&si mple=
Just take whatever screw you're going to screw into the fiberglass and measure across the TOP of the threads to find the dia. of the screw. Then, using that measurement as a guide find what the proper tap drill size is for it. You can usually find a fractional drill that is close to a number drill. For instance, using the examples I gave previously, a 1/4-20 screw hole would use a #7 tap drill which is .201 thousands of an inch. But a 7/32 fractional drill is .218 thousands of an inch and could be used in it's place. You're only talking about .017 thousands of an inch difference and when dealing with fiberglass it's unimportant. A 1/8" hole would use a #38 tap drill which is .101 thousands of an inch, but a 7/64 fractional drill is .109 thousands of an inch. That's only .008 thousands of an inch difference. To put that into perspective, a typical human hair is only .003 thousands of an inch thick. So you can see we're not talking about a lot of slop. Use the chart I gave a link to... read across and you'll find the right size tap drill for any hole you need to put threads into. If you run into any trouble just ask me, I'll be happy to help.
75% thread just means that when you run a tap into a tap hole you drilled you will widen the hole to the point that a bolt threaded into the hole will encounter 75% threads and 25% empty air. That's a normal fit for threaded holes in metals, plastics, etc. The empty air is needed so that the bolt can screw into the metal. If there were no gap then it would bind and score and probably be destroyed. (When you use an anaerobic liquid thread locker (like Loctite 242) on a thread it fills that 25% empty air space and locks the bolt in place)
BTW, Be very careful if you're thinking about using that stuff on fiberglass. It's intended for metal to metal and can either weaken or harden plastics.
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1987 V20 w/1987 150HP Yamaha on a Shoreland'r Trailer
1978 16.5 Airslot w/1996 120HP Force on a Four Winns trailer
1996 V21 w/1993 200HP Mercury on a Shoreline Trailer
All towed by a 5.7L Hemi Durango.
If God didn't have a purpose for us we wouldn't be here, so
Live simply, Love generously, Care deeply, Speak kindly.
(Leave the rest to God)
Silence, in the face of evil, is itself evil. Not to speak is to speak, not to act is to act. God will not hold us guiltless.