I use foam tape weather stripping...for all my light bleeds, just place a strip on the cab behing the marquee and done.
This isnt a new find, there's a thread in here somewhere on using contact juggling balls as trackballs.
The difference here is that this is acrylic, which isn't as durable as the clear polycarbonate ball you would get from happs. Acrylic is softer and scratches alot easier than polycarb, so a few months of use and this acrylic ball will get scuffed up pretty bad. However, this is the reason the same site has polishing kits to remove minor acrylic scuffs.
So yes its good for initial eye candy but will show more wear and tear, hence the cheaper price. The polycarbonate is also heavier ball, so a purist will see the difference in the roll, although it works you will get some slips using the clear ball.
If you plan to light it up, get ready to blind your eyes out, if you fire a light at the bottom through the ball, and hit it at the right angle it's annoying and blinding, and you will have this wonderful dot on your ceiling fromm the magnified light. In thinking though this may be what you want....in reference to your theme.
I had to design a RBG board and modify the shells of my T-balls so the LED would fire through the sides, for a more diffused solid color in the clear rather than a concentrated magnified beam of light.
They do look sweet though, but just and FYI from a member who has worked with a few of these bad boys.
I would recommend, spending and extra $20.00 and getting the real thing from divemaster to complete the quality aspect of the build, less scratches and the proper weight....
http://www.arcadeemulator.net/cgi-bin/shop/cp-app.cgi?usr=51F2166288&rnd=5241423&rrc=N&affl=&cip=99.127.150.14&act=&aff=&pg=prod&ref=Replacement+Balls+for+Happ+Con&cat=Trackballs&catstr=HOME:TrackballsAlso yes, 76mm is the correct size. 76mm = 3".....
I dunno though, i think the UV reactive/white translucent ball would look better in your machine, it would give off a more Tron-esque glow at night....