I fabricated mine this way. I used a steel plate with the holes drilled for the base plate and JB welded a 3" long piece of galvanized pipe to it to attach the flex pipe. Still working great after 4 years. I also fabricated one for my Harman as well, although this already had the attachment for the flex pipe.the quads and heatilators need the little collar piece since the inlet is just a hole on the back of the stove with no way to clamp the hose to it.
i really didn't have the tools (welder) to fab one up. though i think even with a hacksaw, drill and some jb weld, it could be done.
i thought about going that route, but just caved and ordered a kit.