one simple possibility for that forge, if you decide not to build your own, would be to stack up fire bricks (check your local brickyard or hardware store) around the sides to help keep some of the heat in. you can change them around depending on size of work etc. just make sure you leave enough exhaust area to prevent back-pressure.
if you want to get fancy paint the hot faces of the bricks with itc100 or plistix
best price i found for itc 100 is on ebay.