I made a box beam following Nick’s book. I did use an inferior quality of underlayment 19 mm thick. The result is a box beam that is not complete linear. Therefore I made the openings in the frame 1 cm wider then the prescribed 2 by 4 inch.
I draw a straight line on top of the box beam and centered the frames on that line. The rest went as described in Nick’s book.
The box beam is placed on an external strong back. This gives place for equipment like staplers, saws and heatgun.