Slowing down the coal burn

I have, (in a concrete block 16X16 shed ) a pot belly stove. This stove, while lined with firebrick, does not have a provision for gasketing around the doors. As a result of no gasketing, it is difficult to slow down the burn rate of the coal.
I have closed the dampers and the manual draft in the pipe, which helps allot, but i am looking for ideas about slowing even further.

One thing I have heard, is to dust the surface of the burning coal with ash to impede the draft. Similarly I have heard of laying some pea coal on top of the nut that i am using.

What about increasing the draft above the fire to divert some pressure from below?

Any ideas..

My Efel at home can be slowed down to a surface temp of between 200 and 350