If I were to attempt something like this I would start with a piece of PVC pipe. Something around 7/8 dia id. I would drill a hole in the top deck that the pipe will fit into. Next you need a Walmart 1/4 inch thick kitchen cutting board. You will need to heat the cutting board and conform it somewhat to the bottom of the hull (centered below the hole, about 10x10 sq should work. A large bolt screwed into the plate will keep the tubing centered. You then need an adhesive to glue the plate down to the bottom of the hull. Silicone or 5200 adhesive should work. The trick is the surface area if the glue, you can probably pick up a car with that much surface area. The top deck is probably strong enough to support the mast in the tube. If not you may need to cut a 2 inch washer to fit over the tube and against the deck. You can either glue the washer in place to the deck. Just gluing the tube down to the bottom an to the deck top should hold it all in place. You can buy the hobie kayak sail from any Hobie dealer or mail order. Of course you will need a rudder to steer, and will probably need some sort of daggerboard if you want to be able to sail upwind. Be aware that the Hobie is a true sail, nearly every other add on sail out there are all down wind only, so know what you are buying. Talk to other kayak sailers as well, the more you know really helps make the right choices. For me personally It's Hobie all the way, there is nothing else out there that comes remotely close ( because of the mirage drive). Adding a sail kit is just another tool in the toolbox. With mirage drives you can easily triple your range over paddling, With a sail kit you can triple it again. Keep in mind Hobies can be paddled just like any other yak as well. What we do is paddle till we get tired, then switch to pedaling ( all different muscles). If the wind is right we can rest and sail for a while. We do a lot of excursion kayaking sometimes hundreds of miles, I always take the sails on the kayaks.
Hope this helps you