1) Don't burn another fire in that stove until it is properly installed.
2) First step - read the manual that came with the stove. (You can download one if you don't have it anymore.) Pay particular attention to the sections on clearances and venting.
3) Figure out where the stove is going. Then, pick out a brand of commercially manufactured solid fuel chimney pipe. Be prepared to spend as much on the chimney as you did on the stove, and get someone who knows fireplaces to help you if you don't understand anything in THAT manual, which is specific to each brand of pipe.
4) Cut that hole in the roof. Vertical is the only way to build a proper chimney system in a vehicle.
If you aren't willing to do the above, at least go sign a durable health care power of attorney and a current will!