John, a number of years ago I owned a Continental 25 Folkboat that was built in Canada
by Whitby Boatworks. While I did not have a trailer, I do recall
a sister ship which had a custom tandem trailer. As I recall
, there was an angled channel for the keel
(angled as the keel
is angled downward aft) and four adjustable pads. I don't believe they winched the boat onto the trailer, but rather put the trailer (which had a long tongue extension) a little further out so that the boat could be pulled up by docklines from both sides and floated about 6 inches above the trailer. The trailer was then pulled up until the keel rested in the channel, the carpeted pads were adjusted and lines attached to the trailer were then tied down to the forward and aft mooring cleats
before the trailer was pulled out of the water
. For longer trips ratcheting straps were used to hold the boat to the trailer.
I would think that you would need to have the boat out of the water in order to get the dimensions right, atlhough I guess one could get by with a pretty good set of line drawings so long as the boat was floating on her lines.