Where do you buy those super super small leash plugs that are always on longboards, or is that just a string glassed into a hole filled with resin?
In th past, that is how I did it… Just drill a 3/8" hole - fill with resin and then drill a 1/4 hole through it…
BUT I want to do a fiberglass leash loop… Next time for sure. 
If this is what you are talking about, check out some of these threads. Great tips and direction.
http://www2.swaylocks.com/forums/boring-leash-hole-lb-center-box-tool-tips-advice-strategies
After you router out your fin box, like da5id said.
Measure up about 3/4 -1" from the tail end of the routed hole, double check placement by figuring out how box is gonna sit.
I drill from the top (through the glassed side) 3/8" forstner bit, it should lead straight through into the routed hole.
Cover top of board with tape, and make sure it is not sunk in, should be slightly domed so you have something to sand down
Fill hole with the resing you are going to place the box in, set box
When you are done you then take a 1/4" forstner bit and re-drill same hole.
Forstner bits are kinda pricey but they keep the cut against the glass clean, the small tip helps keep it centered and you can go real slow, if you got the right drill.
It’s all been said before, but I’m stuck here in the office so figured I would chime in.
Good luck
Steve
Any way that you could do this with a glass on? Or is it not possible.
It’s not possible to do with glass on’s. A leash loop is the way to go if you don’t want a leash plug.
You mean a “bridge”?
Like this one?
Yes, like that, although there are a few different ways of doing them. I like to build up the roving into a long hump, then drill a hole through it. They look about the same size as yours, but the hole is much smaller, so they have an appearance of being stronger. I don’t know if they actually are stronger, because I’ve never had either version fail. The drill-through version does look a lot cleaner though.