I had the same problem with my feral cats and their little houses. I agree they need two doors and the shelter needs to be either higher up or well hidden.
After a few winters of not enough shelter, and agonizing over my cats not using their outdoor houses, I ended up buying solar pool covers and putting them over my patio furniture like a tent, then using bricks to secure them. I put the feral houses inside there too, so they had a whole little area to hang out inside, and I put their outdoor heating pads in there too.
Each year I expanded it (of course LOL) until we ended up covering our metal gazebo frame with a huge solar cover LOL and last year because the neighbor threw his cat out (he is my foster boy Curly in my other posts) I made an extra little shelter off to the side, using a smaller pool cover. Now my feral cats use both of them LOL.
Anyhow these solar covers do a really good job, they conduct the sun's heat extremely well. Here is the link and if you're looking to not spend much money, there might be some for sale on craigslist (?)
http://www.poolsupplies.com/Covers/solar-covers.asp