Thansk for the swift replies everyone.

The strongback for a Guillemot Expedition Single is internal allright, but i forgot to mention that i will have to have a movable setup (otherwise there will be no barbequeues on the porch this summer :-)

I have considered a full 2x4 made from fir but it WILL bend and twist, so i'm not prepared to go that way.

Will 5/8" plywood do for a movable setup as well, if i use f.ex. 3 supports under it ?

No problems in using plywood, fairly cheap and easy to work with, but can only be found here in 8' lengths, so will it be ok to "overlap" using two lengths ?

There is max. room for a 2x4 strongback, otherwise, as one of you mentioned, it will protrude outside the forms.

Thanks.

Jesper