Thanks Eric. Those dims really helped shore up the shape - the aperture is smaller than I liked in the top reference image, but rather that than the Tric not fit.
I'm drawing the kit up in Sketchup first, making sure things fit before drawing it all out in Illustrator for cutting. Here's where I'm at so far -
And the cut files:
(the upper body holes are access for lights, strap etc in final assembly - just in case)
I hope to have it drawn it up by the end of the week but will probably hold off having it cut until I have my Stapleton tric in hand to finalise everything. I'm not sure I'm going to replicate the latching mech in this one - that might well be for a later date though.