You're cutting holes for intake or exhaust?
I'm going to assume you want it for a passive intake (meaning you handled your exhaust already).
If it was me, I'd put a false floor (takes off about 2.25 inches for your height). I'd get some 3/4" plywood and 1 2x4. I'd have the homedepot/lowes/bigboxstore guys cut the sheet of plywood to the dimensions I need (that of my closet) since I don't have a table saw. I'd then cut the 2x4s into 1 or 2 ft lengths depending on the width of my closet. I'd want it so I can lay two down in line with each other and they'd have a 1ft gap between them. I'd then lay them down staggered (think of how bricks are laid then add as much space in between as you want, the spacing depends on the length you cut them at and how much air you'll need).
I'd then lay little anti-vibration pads on the 2x4s since I'm lazy and just like to put my pump on an empty bucket (dwc). Then I'd put the plywood on top of the pads/2x4s. Yay I have a false floor. Now I'd use at least 4 screws on the plywood to secure it to the 2x4s just to ensure it doesn't move should I get lazy and drag a bucket.
Once that's done, if I wanna be quick about it, I'd just get out 3 or 4" hole saw and make 2-3 holes in the plywood. If I wanna be fancy, I'll buy a floor vent @ lowes/homedepot for $8 and cut the right sized hole in the plywood with a jigsaw and call it day.
Because of the staggered studs, light can't reach the point where your holes/vent are. If you wanna get fancy, spray paint your 2x4s black.