Are you handy at all? If his were me, and I didn't want to replace the old box, I'd make a wall plate out of 1/8" plastic, or hardboard, or wood and paint it. I'd make it a size that was esthetically compatible with the thermostat, and round the edges on a router to make it look professional. If you're not handy, maybe you can have something made up for you by a friend who has a saw and a router.

Oh boy this is ugly. I don't know what I'd do in your case, other than remove the round boxes, install rectangular ones, and patch the drywall...

How would you feel about your thermostats being installed in "wiremold" boxes? The fairly tolerable-looking surface mount ones? You could get a blank round plate with a knockout hole in the center, attach a wiremold box to the front of it...

Paint the round plate, or start with one of the prepainted ones which don't usually have knockouts - just drill your own (somehow, scary as it might sound, I think it's even legal to add your own holes to electrical boxes and plates)

Thanks for the quick replies. I was hoping to avoid the patching and fixing the drywall so I may start with Kudzu9 suggestion first since I should be able to find something inexpensive or even free to start and try that...can't hurt.

Thanks
Tanya

Why can't you get a round junction box cover (plastic) and drill holes where you need them and mount the therm. on it also.

I found some surface-mount line-voltage thermostats at Grainger that look wide enough to hide a round junction box... but they're all fancy programmable ones and cost around $60...

Whatever you cover the box with needs to be fire proof. In other words, not wood. Just get a piece of sheet metal, cut it to size for the box and mount the thermostat on that. You might even find that there is a cover plate already available that will fit that box. Shouldn't be too bad.

If the only thing in that box is low voltage thermostat wiring, I personally wouldn't worry about whether the cover plate was fireproof.

The OP said they have "4 wire LINE VOLTAGE thermostats". I would think that would have to be 240 volts. You are right though. If it is true that it's LOW VOLTAGE, then go ahead with the wood.

Ok since it isn't going anywhere...

First off, they make 2-wire 240V thermostats. Second, I've seen "4-wire" thermostats that didn't open both sides at the same time due to tolerance and overlap, so APPARENTLY it should be legal to tie one side always-closed and put a 2-wire stat on the other side.

Thus, I think the original poster wants This (130KB .pdf) or something similar. I'm just not convinced (s)he wants to spend sixty bucks a pop.

I'm sure SOMEONE makes a wide surface-mount line-voltage thermostat.

I've got another odd solution but it ain't pretty. (well, cosmetically it's fine, but it's extra parts)

Fix_it-
It looks like you are right about there being regular wiring in there, too. I should have re-read the OP before responding.

By the way, do you know what the code says about wooden cover plates on regular boxes? I know that you can buy wooden switch and receptacle plates at the store, but I've never looked at them to see if they have a fireproof backing or if that is code required.