I use two 4x8x3/4" marine plywood, then paint it to seal it, and then bolt them to the trailer. To get the 8'6" legal width, you will have a small gap in the middle.
On one trailer the fender was higher than the frame, so I had to use 2x2 bolted to the frame to raise the height of the plywood over the fender.
If I remember clearly on one trailer I had to use u-bolts, which leave metal on top of the wood showing, but with good carpeting that is not a problem.
And getting carpeting should not be a problem either, as most carpet stores throw away old carpeting after installing new.
Load is pretty easy. Obviously the first one is laid on its side (cushioned) and roll it on to the trailer with the mast ball just ahead of the plywood deck.
Next put the second boat on Cat Trax and roll it behind the trailer. That puts about 1/3 of the boat over the trailer. Then lift the stern and slide it forward. Nice to have padding to slide on.
Third boat requires padded 2x4s on the stern and just aft of the mast ball on the boat that is right side up.
Have someone lift the trailer tongue (son Dave blocks the wheels, lifts the tongue and places a 2x4 under the tongue to hold it up.) I get someone to help. Then with the boat on Trax approach from rear. The bows will again be about 1/3 over the right side up boats. Just push and lift the stern to slide it on, and then lower the tongue. Oh, and be sure the bottom boats are tied before you raise the tongue, or you will have to start over.

I have never loaded four, but If I did, the 3rd boat would require some help to place it upside down on the second boat that is right side up. The fourth could be loaded as above.

On the trailer build thingy, there are a couple of threads on that from about a year ago.., I think in the Open and the Wave Forums.
Rick