I considered it once. But never followed through. Just bought one instead.
My idea was just to build a box with a fan pulling air in, and have the whole thing heated by a light bulb of some sort. Not sure what type of bulb you'd have to use to get the heat up, but I was thinking that a halogen might do the trick. Tricky part would be finding the balance of air movement with heat so that it's consistent and predictable....
After I thought about it for a while, I decided it would be easier to just buy one.... (I realize they're not growing on trees in your area, but perhaps ebay is an option?)
Murray