Hi Peter,
Phill in Springfield, Oregon here. I just started looking into this site again.
I took some pictures of the bunking I put under Lollipop. The trailer was originally under a Coronado 15, so is a bit small for the DS but 15 years and 2 trips to Louisiana and it is still doing fine. I posted a picture of this trailer in the main DSphoto area, and my personal galley on this site.

To build this trailer system:
I marked the trailer cross beams on the boat bottom will masking tape.
Launched the boat in my back yard.
Made a temporary cuddy support from the keel box to the cuddy roof.
Flipped the boat upside down.
Made temporary templates of the bunk supports with cardboard.
When I was satisfied with the patterns, I made another set out of 1/4" ply.
I then made the wedge shaped bunk supports out of treated 2X6's.These supports sit on top of the trailer cross beams.
I then made another set of supports out of marine grade 3/4" ply, only longer this time to bolt onto the face of the trailer cross beams and the 2X6 supports, to keep them standing straight up.
Bolted all of this together and to the trailer.
I then put the 3/4" marine ply across the supports, fastened and covered them with astro turf.
The bow support was only a regular roller until 2 years ago. I made this new cradle by using the boat itself for a mold to make a glass and foam cradle.
The DS is almost perfectly flat fore and aft from the Centerboard pivot to the transom. And in the same area from keel to about 15 - 20" sideways. This makes these bunk support a large area without causing any 'oil canning' or hollows. If I was to make my bunks again I would have them closer together, perhaps starting only about 4 - 5" from the keel instead of the 12" or so that I left unsupported. I would suggest that you at least plan the bunks so that they are directly underneath the vertical edge of the seat tanks. Don"t forget to slather lots of your favorite deck waterseal on all the wood, especially the bunk tops, before covering with whatever padding you prefer. I also waterseal every couple of years the wood that is still exposed. 15years and still looking OK.
Hope This helps
phill