i say U bolt. and a some extra nuts and a washer that fits the U bolt.
tightening up the nuts. and it will cause the 2 washers to bend right around the square tubing on tiller.
or skip the extra washer and 4 nuts, and just go for a generic U bolt, with a washer, and 2 nuts. ((suggest adding 2 more nuts. so the nuts do not spin off / back out on you, threads shear off))
if the washers or U bolt bends as you tighten the nuts, keep on tightening till you think you got them tighten enough they will not bounce loose. (may need to get off tractor a couple times to check) and tighten up. on the first time tiller is used with Q/A after that should be trouble free.
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edit ok re-looking back at your pictures, the edit picture of mine most likely would not work, but just a generic U bolt by itself with a washer and a nut on each leg. and just keep tightening the U bolt and let the U bolt bend how ever it wants to bend.
have done it a few times on some pipe gates that look kinda like a P. and on a few other things. it holds, that is all i care about. bending of bolts well just will have to live with it.