A retrofit kit is just like you said. Usually it comes with a reflector with a socket in it, a bulb, and a remote ballast. Some reflectors are prebent, some are flat and you can cut and bend them to how you want it. On a 29, you probably should go with a 175 or a 250 IMO single lamp. Most of the reflectors are 20 some odd inches long, so you can't really fit more than one on there, and most MH bulbs with a socket are around a foot long in that wattage give or take a couple inches. You could also go with the HQI sockets, but I've never used those although I've heard good things about them. The bulbs and sockets resemble Halogen t-2/t-4 lamps.