May I add the following to the already high quality responses:
The weight must be, as with all boats, on the keel. Obviously you need some support to prevent the boat falling sideways. but on a well adjusted trailer the boat can rock ever so slightly sideways. Make sure the straps don’t “push” the boat in to both the side supports, thus changing the weight distribution.
Make sure the side supports push at the chines where the hull is strongest. Never support your boat under the bilge keels, it is a sure recipe for breaking the floorboard supports inside your boat.
Don’t over tighten the straps, the boat must be allowed to move a little. If the package is too rigid something has to break when you hit that unseen pot hole. Basically the boat will stay in its cradle without straps. The straps are for security only and don’t need to be very tight. They should only prevent the boat from jumping off the trailer/trolley (main strap) and from sliding backward (bow strap).
I have not seen anyone mention a trailer plank? A wooden plank across the hull with the shape of the gunwales carved in the ends. The ends are coated with some carpet to protect the gel coat. The plank is a few inches wider then the boat so that the ends extend beyond the gunwales. The main strap, that runs over the top of the plank, is kept well away from the hull by the overhang. As an added bonus the hull cannot be compressed sideways, it is only pushed straight down.
For mast support I made a plywood board with a pintle and gudgeon that fit over the pintle and gudgeon of the boat. The lighting board is screwed to it and at the top there is a mast support that holds the mast well clear of the top of the hull and the trailer cover. In front of the boat there is a mast support that came with the trailer. It holds the mast well above my car and nothing needs to stick out the end, well, it does stick out a little, but just enough to tie the mast to the plywood support. The support board has two small T-shaped blocks at the sides to keep it steady in fixed position parallel to the transom. The T-shape allows me to store the lighting wire if the board is not in use. The lighting wire is run through the boat. This is much easier then underneath it, for it does not need to be fixed to anything inside the boat. I did need to change the cable though, most standard lighting wires are not long enough to run through the boat.
Like Colin I use a flat (trailer) top cover. On a trip to Copenhagen and back (1000 miles) it saved me between 10% and 20% fuel. In other words this one trip paid for the cover. Less fuel is not only good for my wallet but it also means there is more power available for accelerating and speed. Especially on the German ‘Autobahn’ that is a very nice thing to have. I know this because I did the exact same trip once with the over boom cover blown up like a big balloon, just like Collin described, and I have a habit of logging my mileage and fuel consumption on long trips.