D-tube construction is generally quite strong. Some shear webs would offer a lot more strength for very little weight - you can likely use the same 1mm balsa. Put them on the back of the main spars, running the grain vertically. Double up with shear webs on both sides of the spars if you're really concerned.

Next time you build sand the 1mm balsa to a finish before applying it, you'll get a much more even finish that way.

Good luck,

Joel