The originals are 28mm deep; both replacements I've tried are 20mm deep. Doubling up is an interesting idea, would need a bit more wiring though. For now I'm going to try faster/louder fans in the hope that they'll be powerful enough on low.How big are those fans? I have some doubled up fans out of Proliant servers, they are really loud at full power but pretty quiet when spun down a bit. There was a lot of design research into those fans to move air efficiently.