There is a smaller (.250) version of the old "double" headed front screw that came on some Colt AR's. I think R-guns might carry them. Basicly it looked like the standard push pin but had a screw on the safety / selector side. Thats what you need. Or you can have the detent hole drilled at a quality machine shop if you prefer.
Make sure if you have it drilled it is by someone that knows what they are doing. One slip and the fence can crack. Try and find someone that has a jig built for this. I have hade 2 done by my gunsmith with no problems. You probably will not end up with the little drain hole on the side but that is not a big deal. There are people here that do it if you have to send it off.
I'd just have the hole drilled and the push pin installed. Stokes, the Rifles, Uppers, Lowers, Barrels, Forum moderator drilled one of my pre-ban Colts and installed the spring, detent, and push pin for a total of $30.