Hi, I used 20mm MDF strips screwed/glued together make a strongback. It worked perfectly. So perfectly in fact that I regretted cutting the hole in the forms oversize... I later thought that I should have cut the hole as a tight fit on the strongback as this would have made aligning them so much easier.

I used three trestles, aligned lengthwise, so I had six support points for the strongback. I cut six U-shapes from MDF. This way I could get the strongback aligned horizontally/vertically by moving the U-shapes around before screwing them to the trestle-tops.

The only problem I had was when I had to turn the strongback over, I needed another pair of hands !!