OK, on a proper computer now...
This is an early pic of it, and the winch has since been "packed out" by a 3x3" block of wood, so the strap now pulls horizontally. The crucial mod is that the bit with the snubber, as you can see doubles as the anchor post. Originally that was "free" and a third bar was used to form the triangle between post & chassis. If you can imagine that, the winch post was more vertical, meaning the snubber was at least another couple of inches aft , and that at "full forward" adjustment. Will get another pic of the current setup as soon as I get the wheels back on! (middle of the annual brake service!)
Also forgot to say that if you are buying a new trailer for a rib, be aware that most boat trailers are "sized" (e.g "to suit boats 4-5m") for hard boats. Ribs have toobs & cones poing out beynd the "hard" bit of the hull, so you can usually subtract a good half metre of your LOA for sizing a trailer. (Mine is sat on a trailer spec'ed at "max 4.5m").