I haven't personally done it, but have heard of a few who have done it, and what they encoutered: It depends on if you want to make it just splash-proof (or for, like, incidental temporary dunks, but not for long term, or for depth, etc...). Because if you want to make it truly under-water (to a certain XX ft. deep or whatever), the accuracy tightness level necessity goes way up. Once you add the pressure of underwater at depth, water finds its way in at the slightest imperfections. But if you just want splash proof (or monentary dunks), then no, it wouldn't be as difficult (assuming you are handy with fabricating plastic housing and such).
Another problem people that have done this have to worry about, is sweating inside an air-tight housing. The more air-tight you get it (to the point where you can scuba, for instance), the more you create a green-house effect where sweat droplets can form on the inside. I suppose you can solve this with one of those moisture absorbancy packlets (like the kind that come in pill bottles), eh?
I thought "you idiot, you should have bought it as soon as you saw it", I know the value of them, I could have kept it, or sold my oldest one .........
