There are a few off the shelf iso cabs that just have one speaker in them, it depends if you like the sounds of that or not. I sometimes have a problem with the idea of an isolation cabinet. I mean, it's nice to not have a blazing amp blowing everything out the back wall, but there are a couple things you miss.

For one, there's no way to get any sort of "room" mice because there is no room. Any sort of ambience must be put on artificially, which isn't a bad thing, but your options are limted. I guess you could play back the guitar track into a room with a mic...
Also, if it's just one speaker in a box, you are limited to just close micing one speaker, something I don't really like to do on loud guitars anyway. The "sound" of a Marshall, etc.. stack comes from the fact that there are four speakers being pushed to their limit right at you. One speaker just doesn't do it for me. But that being said, I still close mic and these are just my opinions.

Sorry for the ramble, if you are looking for the one in the box, Randall makes one. Maybe put in a 75-85 watt celestion or something.

Ian

Yes. Good point. No reverb. And One speaker. I have one big guitar room that I use ambient mics in. But I want to track dry when i have 2 guitar players. I do record rythem tracks really dry if it is heavy rock. But many players do bring in one speaker amps, and their feel happens when they are all playing. 2 guitars 1 bass drums and vox. In this back room I also record bass cabs but they generally dont have to be cranked. So what I wanted to do was have 2 closets one for the bass and one for a guitar amps. I just wanted to point them into a closet in the wall. The room these amps are in are very dead also. I have seen other studios like this. And just wanted to know how i could record a rythem track next to a bass amp with good separation.