I've never had a highland plant, but I've randomly tooled over ideas on how to chill a doghouse in the past. One thing that may or may not work, someone better versed than me would have to comment, would be a version of the redneck air conditioner. The basic principle is that you take a cooler with the built in cup holders and cut two of them out. You put a small, cheap fan blowing into one and you put an appropriately sized 45 degree pvc angle on the other. You turn on the fan and it blows air across the ice you fill it with an out of the 45. You can turn it whichever way you want to blow the cooler air.
It seems like if you were looking for a cheap method to cool a small space, you could do this with a computer fan, some pvc pipe, a thermostat, and a couple of containers from the dollar store. You could have a couple of pipes running from the tank to a dollar store Rubbermaid container. You'd put the Rubbermaid in the freezer to make block of ice and while one is in the system the other would be in the freezer. The thermostat would tell the pc fan to turn on and it would suck air from in the tank, vs warmer outside air, and blow it across the ice, up the other pipe, and back into the tank.
Granted this is all a little sketchy, but it seems like the idea might work. You could use ice cubes, which are cheap, instead of having a large block of ice and it would last longer if the "cooler" was more insulated, but the idea seems sound. I'm interested to hear what anyone else thinks of this. I know it's not ideal, but would it actually work without too much effort?