Peter, actual egg cartons will do almost nothing except break up high frequency reflections related to a fairly narrow band of wavelengths. If you are just using for an iso booth, then make the walls non-parallel first, just by a few inches with drywall. This can effectively kill off most of the room mode, low frequency standing wave issues that happen with parallel walls.

Then you can get some acoustic foam - www.foambymail.com is cheapest place, and I would also get some of their corner bass trap items. Buy the deepest wedges that you can afford.

HOWEVER, a more useful and dual purpose thing you can do is also use the thing as an actual closet, at least along one wall, and better . Do a two-tiered clothes closet and fill both rails with your fluffiest clothing. This will actually have a whole bunch of absorption as it's quite 'deep' - think of the spaces between the clothes as the same as the spaces between the wedges in acoustic foam. Depth is king as it pertains to generally absorbing materials like clothing, or acoustic open-cell-foam. Even better use two walls, that join at a corner. If you feel like it, put the foam wedges on the other walls. Plush carpet on the floor.

The thing is, you really don't need to worry about it being a booth - sealing off from the rest of the room, unless you are monitoring over speakers while recording. Close-miking, in front of the fluffy clothes, will work wonders, even with the closet door open - preferably with the door open as that eliminates a big part of the reflective surface for the wall that the door is mounted in. This is true as long as you don't have loud stuff going on in the main room. If you can control that a bit, then you have a much more useful closet than just one for recording purposes.

-Scott

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