Just a few things!
On the TT the wing LE taper does not add washout but actually the opposite. Do a check on the aerofoil section on the tips Vs the main wing and it will be obvious.
yes the undercamber tips do add extra lift BUT it is an area of the wing that has little lift at slow speed anyway.
The main thing about the open tips on the underside of the wings is weight saving in an area where there is not much to be gained in strength or in lift.
You can enclose the wing tips using covering film or the like or extend the underside sheet to the tips. In order to not add too much weight you could consider removing all of the paper on the wing internal surfaces and add a wooden dihedral or wing jointing brace to better secure the wing joint.
You will find the difference in flight is not that great and hardly worth the effort. If wanting a faster bird then reduce the wing thickness, cover the wing underside, and add flaps for landing.
have fun!