ARCTIC P12 Slim PWM PST - PC Fan, 120mm Case Fan with PWM Sharing Technology (PST), Pressure-optimised, Quiet Motor, Computer, Extra Slim, 300–2100 RPM - Black

Since the OP is unresponsive, may be this will help someone else... Assuming that the bottom fans are 120mm x 25mm, 120mm x 15mm fans will allow 10mm more clearance (funny how math works)...