Thanks all for the varied suggestions. Time, access to materials, and lack of welding ability knocked out several suggestions. Struck out at the Depot for a suitable substitute, but did pick up a 5/8 bolts, drill bit, and tap. Plan on coming up through the hitch from the bottom just before the extension ends, threading the hole, putting the receiver extension in, and then tightening the bolt to wedge it up tightly (with the standard hitch pin in use, too). Will use a nut on the outside to lock it in place. Might do two, for extra support. That's what Swagman did on the carrier end of their rack. I will check out the local auto store for leaf spring brackets before I go for it.
If someone thinks the above is an unsafe idea (significant weakening of the hitch, etc), stop me now before I drill!
I did pause before buying the materials, but the hitch was ordered from the factory, for us to use specifically for a bike rack, so I don't feel remorse about modification to it (it's rated at 3K, but my boat is 3.5K, so I couldn't triple tow anyway...). I'd like a hitch vise, or one of the other suggestions, but not enough time to order.
This not a bumper mount, but using a real hitch, so the warning in the manual does not apply. Personally, I'd never bumper mount.
Ground clearance is 18", so good there.
It doesn't move forward and back, just up and down (maybe right and left too, but didn't really check), so that warning doesn't seem to apply, either.