Flattening transom skin before installing core.

[Xcomunic8d]

Do you have a t bevel gauge?
Try to cut or sand the angle into the transom base should help keep the shape. And round the edges some.
PL premium adhesive was a lifesaver for me. I used a lot of that in various ways.

2x4’s with thread all as clamps
2x4’s braced against outside so you don’t lose the shape. Be murder to put the cap back on...

Plenty of clamps around the edges. I have tons as a wood worker. I suggest putting 1x4’s between them and clamping down to keep even pressure.

I tried to put no holes in my new work. But you will for bracket, anchor bolts etc.

Good luck. I have plenty pics I can share and many very knowledgeable guys here on all things seacraft. You are in the right place.