make then on the crown board and ensure you square them up to the edge of the crown board before you finally drive the nail all the way in
i have a crown board with four right angle metal L bracket fixed to the corners....assemble the box on this board....all square, glue and nail up..leave for five minutes...done...all square...with two crown boards , i can make ten supers and hour
will it matter yours are 5mm out.....not really, you can use a national super on a comercilal box and they are 6mm different in dimensions
have you tried sitting on one corner with another on the the ground to move the lozenge shape squarer...then put a couple of extra nails in
also Th**nes instruction are bad, download the maisemore pdf on making them up. it is an easier method NOTE Caddon, easipet and other use the maisemore method rather than thorns method
it is basically , make up the two large sides, with the two top bearers and two bottom bearers into an open box, then push the smaller sides in them so their frame bearing edge ( with frame runner fitted if Th**nes) is the thicknes of a frame top bar plus 1mm below the top of the top bearing bar /large sides..i
you juste push it /knock it down so a top bar just is below the leval of the top of the large side.(ie you dont really need to measure just use a topbar of a frame as a template so the bearingedges/runners are in the correct position rather than measure before you fit everything as per the Th**nes instructions