>> Problem: My aluminum boat trailer is rated at 500 lbs.
Are you sure about this load rating? The Trailex aluminum trailers I have seen are NICE and folks have loaded them up with all sorts of boxes, bike racks, etc.

>> Solution (?) I have a 4 X 8 foot utility trailer rated at 2000 lbs that I used to take to the beach before I got my Hobie. I hope to convert it to carry my H16. Has anyone ever done a project like this?
Cat trailers typically have a much longer tongue than utility trailers. Make sure you have sufficient clearance between the bows and the tow vehicle for tight radius turns.

>> 1. Follow the general layout of my existing cat trailer and attach 2 treated 4 X 4 in. beams (about 8 ft long) across the front and rear ends of the trailer bed, and then attach rollers to the beam ends for the hulls to rest on, OR...
This will work.

>>2. I have removable sides I made for this trailer using 2 x 4's for the frame with 1 X 4's attached to them as the siding material. The assembled box is about 3 ft high, and has an optional top made of 1/2 in. plywood. After packing this box I could lift the boat's frame onto the top of the box. This would mean the boat would be resting on its frame, with the hulls hanging unsupported.
Many A Class cats are trailered this way but I have not seen it done with a H16.