The best source that I know of for the actual plugs that fit into egg crate is boston aquafarms (add a dot com if you want their site). I also know that thatfishplace.com sells them and other fragging tools if you are looking to pick up some other stuff while you are at it.
We've thought about trying to make the plugs ourselves, but the problem comes in getting a mold that will release the concrete. Silicon seems to be optimal and I could easily get my hands on a good amount of that which could be cured in an oven, but forming the silicone in the first place without wasting massive amounts of it is the hard part. Also, I think you would need a vent in the bottom of the mold (where the plug part sticks down) so that a suction is not created when the plugs cure. In other words, conceptually it is not hard to do, but it seems like in order to make a good mold for the plugs you'd have to have something machined. This would make the price prohibitive unless you could sell a pretty good number of them, which of course you couldn't do without starting up a business and probably getting handed a lawsuit for copying what I can only imagine is a patented design.
I always thought frag plugs wouldn't be that much of an expense, but I cruise through a couple hundred a year probably... so it doesn't take long for a bag of concrete, some aragonite sand, and $1 ice trays to pay for themselves. If I could get the benefits of a solid plug without paying out the nose to make a mold, I'd be tickled pink.
