OK, I've found my MOSFETs:
IXTH40N50L2. They're designed for linear operation so they're ideal for this. Two of them will be fine for 300W operation, I think I'll have a max voltage limit of 100V. They're TO-247, so they'll fit nicely under the cooler as previously discussed. There were a couple of alternatives from the same company, but that's the cheapest one I can get from Farnell UK which has a minimum order cost of £20 rather than £50 like everyone else.
On the subject of cooling, as SeanB said, the copper heat pipes won't make great contact without a copper plate underneath them (thermal epoxy again). What about the back side of the MOSFETs (between them and the board)? Options as I see them:
- Cutout on the board, glue the MOSFETs to the copper plate;
- No cutout on the board, glue upside-down MOSFETs to the copper plate;
- No cutout on the board, MOSFETs sandwiched between back brace made from insulating material (e.g., FR4) and copper plate.
I'm currently investigating hardware over-voltage / over-current protection (watching Dave's video on multimeter input protection seems like a start).