I had one on a previous trailer. It was a custom addition, simply a length of metal box section, with a slightly smaller box section inside with hitch on it, under the existing drawbar. At the launch site, unhitch the vehicle, wind the jockey up a bit, pull the extension out and hook it back on the car. I used spring loaded pins (intended for gym equipment) so the extension snapped into place when I pulled it out far enough with the car.
it worked well enough, but only in some situations. It only added about 3m of extra drawbar, so you still need a launch site with a reasonable slope. For my specific requirement with that boat, that car and that site, it worked. Where I am now it would be useless, and I use a tractor. Also doesn't solve the problem of slipping backwards. It just allows the car to stay dry, if its only "just" being dipped in, in that particular situation.
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