Try finding a cheap doghouse for a cairn terrier sometime.
The closest I could come up with was an igloo cat house which cost just about as much as an igloo dog house.
I think we can do a little better than that.
Find a cheap, sturdy storage container that has some holes around the rim which will serve as drain holes for water.
Measure out and cut a piece of carpet for dog to lay on.
Cut a hole in one end. Rotozips work really well for this. Do it outside though.
Glue the carpet to the bottom of the lid. and weight down using container top stacked with books for weight.