I would probably make separate fiberglass forms and then join them. First, I would make the cradle for the hull (using the hull as a form with plastic on the hull, ,carpet, a little more plastic, then fiberglass and resin). I would then grease up the end of the cattrax tube with Vaseline, wrap and tape some 3 or 4 mil poly sheeting around the axle with 1 wrap nearest the end. I would wrap some glass around that and let it cure. Slide the glass off the axle, cut it in two, and now you have two half forms for mating to the axle. Glue those to cradles with a lot of thickened resin, tidy it up, and let it cure. Then glass some more to make the joint strong. Done.

Alternatively, you could tack the half tube to the cradle with super glue or quick setting epoxy and squirt in some hardware store insulation foam around the joint. Carve this to a good shape and then fiberglass. Do not rely on this foam to be structural, though. Use it only as a form to put the fiberglass on...and then put plenty of glass on the joint.

For the trailor I've taken some 2x4s and milled them between my band saw and table saw to fit the shape of the cradle and have a V-notch to receive a metal tube (stainless). I lay that on the formed cradle, with the tube, thickened resin between everything, and then vacuum bag several layers of woven knytex glass fabric to that to join in to the cradle Then I cut the ends of the glass around the metal tube, and insert a rod in there to act as a pivot. With a couple of metal fittings bolted to the trailer, these have been going for about 8 years strong and show no signs of wear or loosening even with double stack loads and a lot of miles. They align themselves perfectly to the hull when loading or trailering.