I've got these
(or something similar, anyway) fitted to mine; only real problem is that the "L" shaped square tubes are fairly soft, which means if you nail them while loading the boat, you have to reshape them.
Other than that, they work pretty well. Still tricky loading in a cross-current (like on a slip perpendicular to a river.) The posts by themselves are not strong enough to keep the boat centered. Luckily, I don't do that all that often.