Thinking about building an isolation booth, Any advice or pitfalls to watch out for?

  I have a small but sound-proofed and acoustically treated (Aurelex studio foam and a ton of leanard bass traps) rehearsal/recordng studio.  It's only about 11.5' x 15' x 7'ceilings as it is literally a free standing room (4 walls/ceiling) within a garage.   Up until now all tracking is done right there in that room which also holds my desk where we monitor everything so dialing in mic' placement on a 4x12 cab is a record and listen back process because it's too loud in the room to hear the monitors and I don't get enough isolation from headphones to keep the room noise out so I can't make the call that way.  I have one corner of the room which isn't used much other than to stack our guitar cases.  I am thinking about building a small Iso booth  maybe 3'x4'x7 high.  So that I can slide a cab and mic in there, shut the door, keep the head outside & actually hear hat the mic is hearing.   know in a small space I would lose much of any room sound capabilities, but to be honest my room is pretty dead in that there are almost no audible reflections at all anyway so I don't think I'd be giving up much.  Think 12x15' vocal booth.... I probably should actually brighten it up a bit!
 
  What do you think?  Is it worthwhile or just a waste of time and space?
 

I appreciate the info there Batsbrew, and while it would be cool....  I don't thing I can make it much bigger for one (and still leave enough room for the band to rehearse in too), and I currently only have a wall unit a/c permanently mounted in the front wall (across the room from where this would go)  which I generally shut off while recording vocals in the room.   So while that may make it a multipurpose space for recording it will really cramp our practice space.   That's why I figured I'd make it small and when not in use for recording it will basically be a closet for the guitar cases. and if it's just the amp cabinet in there then I don't need to worry about building an HVAC system into the design.

I more or less am hoping to find out if a closet type space this size (treated of course) is an effective environment for tracking guitars in?
Should I put commercial type carpet in there like I have in the rest of the room or put in a few square feet of wood floor?

Am I setting myself up for relative success? or an audio nightmare?!

Home-made Movable (folding) ISO Panels. 

These can be fixed-hinged 4.ft from the corner (where you suggested), when in operation simply unfold (open out), when not in operation simply close it flat against the wall. They can also be bi-fold to create an almost complete enclosure (ISO Booth). 

This idea is great for small project studios, allowing for times when you need that extra space due to movable (folding) panels. 

If you want to get fancy? insert a window, good for giving cues. And pulling funny faces and whatever (joking) LOL!! 

And on the wall an in-wall mount for a transport controller (perfect for those lonesome times) also for mic cable connectors and DI. 

If you end up building the dog house, avoid using 90 degree corners and/or parallel walls; this will help to minimize standing waves and boominess.

http://www.vocalbooth.com/products/vb-guitar-amp-enclosures/