Thanks for the reply.
I have already cut two 120mm holes in the opposite side cover, and used some rubber stripping to seal off that side of the cage vertically. Hard to explain, but in essence, two 120mm fans now blow air directly onto the drive cage, directed fully at the drive vents (the air from them goes nowhere else in the case) and the fans in the factory locations now all do evac. Temps haven't changed hardly at all.
I am going to flip the rear 120mm fan so it is intake also, as my CPU and SATA controller temps have gone up, likely because they only have access to hot air coming out of the drive cage. I also cut four 1.5" vent holes in the rear of the drive cage, next to the SATA backplane, hoping it would free trapped heat. Instead, I think the rear evac fan is pulling hot air from the drive cage, which, with the two factory side fans doing the same, I believe is undesirable. I am going to put some tape over the vents I drilled to see if that helps a bit.
I've done about all I believe I can do, aside from trying different fans. Nidec Betas or Delta fans would be ideal, however this server sits inside my entertainment center (the cabinet is vented with four fans) and fan noise is already audible from the couch. I could go with slightly louder fans and not have too much concern, but 3000-6000 rpm fans are definitely going to be an issue.
Was hoping that, in addition to some DIY ducting, I could get my hands on some legit high pressure fans that could muscle the air over the drives, without raising the noise floor too much.