I picked up one of these a year or so ago...for $25

http://www.elitedeals.com/npl-1400.html

Simply stack your chimey to the outside and let it use internal air to
supply it. Few shops are tight enough to prevent enough O2 in to keep it
burning well.

Ive been burning cut wood scraps from an old building that I tore
down..well..cut down with a chain saw

good deal. you may not need it in your mild climate, but adding a heat
exchanger wood pun give you 50% more heat. A word of warning, the
more heat exchanger you have the dryer your wood has to be. You need
to run the fire hotter too. I know of a guy that puts a pint of used
motor oil in a quart oil bottle and toss it in the fire, burns REAL
hot.

Karl