On ebay, cant think of name, maybe gator trailers, have awesome price on parts...fenders, hub to hub...etc
Put axle around 60/40, 55/45. meaning 60% from tongue=20ft trailer, axle centered 12ftish mark.
If you can, use side rails being one piece, follow contour of boat, and at the bend, cut pie shape out, fold together then to close pie cut, and reweld there. Cross supports, go 20-30 degree on chop saw, so then boat sits inside trailer. aloud up to 108in from fender to fender, so b78-13 tire, with 6.5-7in fender witdth, gives you around the 7.5-8 ft axle measure.
Aluminum, have to tig, use 2x3, 2x4 channel, turn outward so marker lights and taillights are sunk in frame,..LOOKS KILLER., plus WAY less $ than aluminum, but you have that kind welder..NICE, to aluminum...good luck