Are you concerned with isolation? If so, just one layer of drywall will do very little. In fact, if isolation was a consideration, what or how did you build this. Come to think of it, with windows in the room I guess its not a priority. However, if isolation IS an issue, let us know and we'll tell you how to improve it.
As to the treatment, how thick is your fiberglass? Very small spaces need all the low frequency absorption they can get, and thickness increases the absorption coefficients at low frequencies. You can see here.
http://www.bobgolds.com/AbsorptionCoefficients.htm
Also, use spacers behind the panels to offset them from the walls, leaving an airgap between the wall and panel. A 2" thick panel spaced off the wall 1" will have about the same coeffiecients as 3" thick panels.
I would also straddle the wall to wall corners, and wall to ceiling corners, as room modes terminate in the corners. Actually, if you have enough, or your budget is enough, personally, instead of straddling corners at the walls, I would use "superchunks" of 4" thick rigid fiberglass. They are triangular shaped pieces cut from the panels. Cut a panel in half, and then disect the 2 pieces from corner to corner, which gives you 4 pieces per panel. Stack these from floor to ceiling and cover with fabric. These are great "bass traps" and have been tested. Here is a plan view below.
And also suspend a few panels as "clouds" on the ceiling. Yes, this will create a pretty dead space, but for small rooms, you won't get great acoustics anyway as far as ambient sound is concerned. Just spice with reverb/fx to taste.
fitZ
