Here is a (slightly blurry- sorry) picture of the mini leash cup. Again- this is the SMALL one. I am using this one for a place to clip a small biner to only. The larger one would be better for deck fittings because they have a bit more room. The cup is designed with a larger diameter at the bottom than at the top. You drill the hole out for the larger diameter and therefore it naturally leaves a small gap for the epoxy fillet on the deck side of the fitting. The brass pin is significantly lower than the deck height so you have plenty of room for sanding to the curvature of the deck. This one was sanded quite a bit after installing and there's still plenty more room. Some have a molded plastic pin, some brass, and some stainless.

These are available from any surfing supply- fiberglasssupply.com greenlightsurfsupply.com and foamez.com just to name a few. The standard size is between 1 1/8" and 1 1/4" The smaller one is typically marketed as "mini leash cup" and is roughly 1" OD. I can't remember for sure. The one in this picture is the mini leash cup. I don't have any boards right now with the larger one to take a picture, but it's the same thing only about 1/4" larger and on some models the inner hole is round where this particular one is squared off on the edges where the peg is. Again, they are common and you will find a few small differences. The line in this one is around 1/4" diameter and has just a little room to wiggle.