What is the wing span of Boxer 45?
I only have seen 60, 90, 120.

I think you can skip the L shaped spar for normal smooth flying.
I have not built the Boxer, but had Boxer 90 cores and read the instruction manual.
My plan for the spar was to use carbon strip of 3/4 or 1/2" width. but not L shaped.

however, you won't get much trouble with the spar if you plan to do lots of snaps.

I wish I didn't sell the core. If I get a chance to have one, I will be building it as fixed gear with larger control surfaces.