Waste Oil Forge Dimensions

I'm just getting into blacksmithing/forging and the cheap (free) cost of waste oil has me working in that direction.

My first attempt at a waste oil forge worked to some extent, but I determined I did not have a thick enough refractory lining.  I also located the burner input centrally, which created a cool spot where the fuel was being injected into the forge -- which is precisely where you don't want a cool spot.

The chamber dimensions of this forge were 8 inches diameter, 9 inches in length with the work opening of 4x4 inches centered in the tube.  I'm using a siphon nozzle setup which pushes compressed air through a .023" mig tip and a 5/64" hole in the plug which is the burner tip. The design of the burner is based on this:

http://www.youtube.com/watch?v=E-l21wQ6X-E&feature=youtu.be

The blower I use is one I removed from a power vent water heater.

I ran the first forge on kerosene initially, but to get the heat I needed from it I ended up with a flame roughly 2 feet long shooting out of the forge.  I never switched over to waste oil due to the design flaws in the forge.

What I'm trying to do with the current design is keep most of the flames inside the forge, be able to burn waste oil, and reach forge welding temperature.

The design I'm working with now still has an 8 inch diameter tube, but I've increased it to about 11.5 inches in length and I've got enough room for a full 3 inches of refractory in all directions.  The burner will be located at the rear of the tube to avoid a cool spot in the center.

What I'm wondering about is the appropriate opening size and location for inserting billets for heating.  The opening I have planned now is 4 inches wide by 3 inches tall and located  near the bottom of the tube (which is horizontal). I plan to completely seal off the back end. Is it better to have a small opening like I have planned to help with heat loss or will that potentially create combustion problems at that size?

Sorry, but I have no pics at this time.