Done it for a beneteau
I purchased a demo alum. trailer for my Beneteau f235 wing keel with many of the same questions you have. There were a couple of things I did; I contacted an owner of the same model and asked him to take measurements of his boat trailer for me. The important measurements went as follows: distance from bowstop to first bunk upright. distance from bowstop to second bunk upright. distance from bowstop to 3rd bunk upright. distance between all uprights. Angles of uprights (if any) Distance of front axle to bow stop. Height of all uprights from the plane of the keel pad to the top of the bunk. I was able to modify some hefty dock coupler parts to clamp onto the 3x3x.25 alum. crossmembers and used 3/4" set screws to make fine adjustments to the height of the uprights. This adjustability allows me to paint the bottom while on the trailer by raising and lowering the bunks as needed. It also allowed me to make adjustments when I went to get the trailer. I used 1/4" wall galvanized pipe for my uprights with alum angle iron for bracing. It isn't the prettiest trailer on the road, but it made the trip from Racine, WI to the twin cities with nary a hitch. All that said, after I bought and modified the trailer, I found a place that would make a custom one for only $300 more than I spent. Sorry, but I have lost the name of the place.
