Assuming you're doing 20L batches, that's not a lot of thermal mass.  Meaning the liquid is going to be more prone to swinging with the air & 10 degrees is huge swing.  Even a well insulated plywood box would be helpful, but you can't beat finding a cheap/free fridge & putting a temp controller on it.  
Another option is to increase the thermal mass, like getting a large plastic trash can (like those 100L+ ones you find on building sites & larger commercial kitchens) & adding some cold water - but not so much you float the carboy!  The extra thermal mass will act as a buffer against ambient temp swing & for an added bonus you can freeze some water in plastic soda bottles to help keep it down further.  Just make twice as many as you need so you have a set in the can ( 

 ) and a set in the freezer.  If you use the smaller bottles, you'll need more but they'll freeze faster & give you more surface area when they go in your can ( 

 )  Swap em out once or twice a day & you won't believe the difference it'll make.
You could even make a 2nd vessel out of a cooler & use a cheap pump to circulate for even better efficiency.  Keep the frozen bottles in the cooler, drill a hole for a outlet at the bottom of the can & return the water to the top (heat rises).  There's all sorts of variations of it depending on how inventive you are & how much you want to invest into it.
Lee
"Show me on this doll where the internet hurt you."
"Every zoo is a petting zoo if you man the fuck up."

 BN Army // 13th Mountain Division 
