Just a thought but you may be able to rig up a larger enclosure for them by using something like a chicken run (something like this; or one with a roof like this so it'd be easier to walk around in and harder for them to climb). It'd probably need some added catproofing especially around the door, and I think setting up a second, smaller one attached to it to make an airlock style entrance would probably not be a bad idea. That way you could go in through the exterior door, close that, and then open up the interior one without any risk of the cats being able to slip past and escape.