Basically if you cut one side approximatley 12" extra to the height with perfectly parrallel sides, ( and squared 90 degree corners ) and work from centreline dimensions, draw and cut out the tapered shape. You are left with two wedges each side.

Now if you did well with trig at school you'll realise that the rake of the angle of the wedges will be exactly that which you need to slope the throat horn back VERY accurately.
So, laying the pieces on edge on a good flat surface ( e.g. a sawbench ) attach a plate and some 1" flanges and you end up with this;


You can clamp the jig to the bottom accuratley, glue and clamp the throat horn to the bottom like so ( I wrap a 4" strip of shrink-wrap on the jig along glue lines to prevent accidental gluing to the DR!!!!! );


Then, you can turn it over and get great access for screwing the horn throat to the top ( 5 screws ) without it shifting

THEN - I attached the ONE side ( yes I know the woofer baffles aren't in yet ) with 3 screws BUT DID NOT glue these, using some scrap wood jigs the right height and notched at the top, sat this on the saw bench, screwed the other side THEN attached the bottom;

The result - a very accurately square cabinet with an accurately and easy mounted throat horn.
After the glue dried, I unscrewed the unglued sides ( the holes will be good for when I finally glue the side on anyhow ) and made and attached the woofer baffle
Maybe you all have easier ways of doing this but this system really helped me with the DR300 assy.
Harley
edit aug 17 - fixed some typos, added sketch and ammended the bit about the side to say DO NOT GLUE the sides