Very very nice build up man!
I also thought about adding UVB to a COB light. Then i came accros IR, then maybe more reds, then some blues came in my mind and finally some UVA to make it complete haha.
I ended building my spaceship and ordered a UVB tube that also needs to be fixxed.
The problem i came acrros seems the same you are facing atm. "where the fck do i put the fixture for that tube without creating a shadow?".
In my build i got a similar setup in terms of COB position. I got 5, 4 in the corner like u and one in the middle .
The only solution i found was getting 2 of them and placing them paralell in the middle with the center COB between them like this:
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