Very nice! Simple effective design, and it looks good from the outside.
Orchidtatter, I made one just to keep the cold air from building up in our bay window and damaging the plants sitting in it. (We also live in Minnesota.) Just get a computer "muffin" fan (they're 12 VDC) and an old power adapter from a phone charger or other discarded electronic device. They all have voltages printed on them, anything from 5VDC to 12VDC will work. The higher the voltage, the faster the fan will spin (and the louder it will be.) It doesn't take too much air movement to keep the plants healthy.
I had been considering building something like this but my wife came into a small lump of money so she bought the
Grand Cayman and saved me the trouble. Can't say that I blame her.