Advice please.. which tools and in what order to make non straight mullions/muntins

[rudy de haas]

I need to make a large (44" wide, 82.5" high) door that rather seriously exceeds my skills. The two drawings below are by Heinz Demes, a stained glass maker. I need to make the frame.

I have the yellow birch for this coming, making the basic four sides should be within my skills, more or less. However, the "tree" parts inside have me worried.

Should I make the tree parts first and then cut the mortise/tenons to put them in place? or should I put them in place using rectangular pieces and squared edges, then cut them in situ to produce the right shapes using a jig saw, router, and sand paper? or is there a better way?

Regardless or the order in which things are done, can I use a mortise/tenon join for the tree root if that combo includes drilling part of the mortise into the tenon on the bottom horizontal piece (and thus into the mortise on the vertical side)? Since that bottom strip will be about 11" (the drawings show top and bottom the same, but I plan on a 4-5" top/sides plus an 11" bottom), one idea would be to tenon it only on the bottom 8" or so (maybe 2 x 3.75" tenons?) and thus butt join the upper 3" where the tree root would then act as a tenon holding both the root and that horizontal piece to the vertical. Or, is there a better way?