Can you please post a picture of the underside of the unit where it mounts to the riser. The unit blocks the view so it is hard to see how it is mounted.
In the last pic, The part in my hand bolts on top of the stock riser,5/16x 18 bolt. The top of the riser is already threaded. The other piece is on the unit, it is upside down in the pic. It bolts to the unit with a teflon washer inbetween so you can turn it but still be snug. It slides over the part bolted to the riser,put a quick pin between the two and it makes for easy removal of the unit. Hope this helps,Im not to good at explaining things. The main thing is to find 2 pieces of pipe or tubing that will slip fit into one another. I just used some stainless pipe I had laying around. You will probably have to get one of the pieces machined to get a slip fit. Weld a round plate on each one,drill a hole in them to bolt to riser and unit.
MEMBER CRAPPIE MAFIA