At those dimensions no need for a centre rib
my old b4 cause it was ali every one thought it must be heavy...in fact it was with large radio gear and lipos over 400 grams under weight
a bit too much realy...should have left it hanging underneath like you have,
mine only took two panic strichened evenings though
but will try harder next time
p.s did you see my carbon yokomo ( Rebel cc4 )