Hey, didn't you end up with an Onan like mine?
Word of advice: Don't fawk around with any kind of stand or mount or any of that shit. Just cut a big hole in the floor. At least one hole on the intake side and one hole on the exhaust side. Make them the same dimensions as the openings in the bottom of the case. Then route the exhaust pipe out to the side of the chassis, leaving it about 2" proud. Bolt the generator to the floor either from the underside into the mounting holes, or from the top side using the shipping clips and some lag screws.
I built a big fawking mount kinda like what you're working on. It was a waste of time.