I've been considering your grill problem and really don't think modder's mesh would blend with the design of the piece. But if you're willing to go with holes that large have you considered hardware cloth with simple semi-flat black paint? Hardware cloth could be cut to fit the rad openings and attached to the rad frame rather than the side piece. Those cut ends, if left long instead of trimmed to the cross-wire, make good inserts. Other than that speaker grill material has very little air resistance but might be a pain to keep clean and I doubt you can find modern grill fabric in the right color (didn't you research that the last build?).
I also though about the design you used. Not bad but I might have picked up the other openings in the front panel instead of the dominant one. Those four side openings with the offset horizontal break and squarish bottoms might make an interesting pattern over the space of the side panel. No reason I know of that the curved part of the pattern couldn't stick up above the rads so you wouldn't have any air restriction. Probably be a lot tougher job to cut though. Just a thought (and please forgive the bad Paint copy/paste image - they should all be the same height/scale).
I won't say "Keep up the good work" because I know you will - but do keep up the great posts! Seeing your progress as you've laid it out is an inspiration to many, including me ...

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