That gizmo you found is the same type of thing that I have on my Force. The Force manual calls it the 'trailer lock'. I'll show you it in operation next time we meet up. Unfortunately, my trailer only has tiny 'ickle 8" wheels, and with the engine lowered onto the 'trailer lock' there just ain't enuff ground clearance for my peace of mind (at least not on the roads I travel), so I tow with the engine fully up on the hydraulics. Never had a problem, even on the track from NEB to Portlogan, and thats certainly bumpy.
I always meant to build a copy of the 'trailer lock' so I could put both on and get some decent ground clearance, but haven't got round to it yet - I keep getting sidetracked by folks wanting to try out their new boats etc!
Anyways - when ya next taking your tub to NEB?