I came up with a solution.  I will use a length of wood as a “strap”, across the stiffener, to hold it in place while the thickened epoxy is curing.  I will use a cleat, like was used to temporarily attach the bottom to the shell, drill 2 holes in it, and thread a length of steel wire through the holes.  then with the cleat on the outside of the shell, I will thread the wires thru one of the cut holes for the door, near the end of the “strap”.  I will use such an assembly on each end of the strap, then tighten the steel wire to hold the strap fast against the stiffener.  should work.  no additional holes needed.

as for where the beveled end of the stiffener contacts the panel 3/4 joint:  I will sand the edges of the fiberglass tape smooth, but I will leave a thin layer of it so as to not weaken the joint.  then I will use an angle finder ($6 from Home Depot for a plastic one) to measure the angle on the inside of the door, at the panel 3/4 joint, and transfer the angle to the end of the stiffener that needs to be beveled.