I used two black ABS pipe couplers for 3" pipe, that are attached to each side of the cabinet. The inside is a little larger than 3.25" so it seems to fit the gun alright. It's not as snug as some other ways, but it seems to work well. I'm still thinking of lining it with felt.
I've heard of others that have made holsters out of leather. Some have simply cut a hole in the front or side of the cabinet. Also, I remember someone else using a 2x4 framing strap dipped in latex.
I don't think I've heard of anyone using a retractable cord, but that might be possible.