[WNC Goater]

I DO UNDERSTAND the attraction of ready to use, "out of the box" and not dinking around assembling parts and building yourself. I've looked at ready made and like you, mostly find square. Nothing says a square forge won't work. They will, and lots of knifemakers use them. NC Tools makes some nice forges, and even have some specific to knifemaking.
But as easy as it is to build a forge, and burners, to my mind it makes no sense to buy. Not when you can build to your specs, rather than settling on what's available. Mine is made with plumbing parts from Lowes and a piece of stovepipe. You can make a very short, tight chamber or longer with more volume to handle longer steel.
As I'm sure you know, you just have to have something that will get up to welding temperatures for damascus so yes, a certain amount of efficiency is necessary...which goes a little beyond just heat treating.

Heck, I've built a coal forge out of half an old air tank, bottom tuyere is a 1 1/2" pipe with holes drilled, the firepot is "V" shaped and formed with dampened kitty litter/sand. I use an air mattress inflator as an air supply. It will melt steel. It works great and enables me to heat specific parts of the blade rather than the whole blade. You can just build a box, fill it with dirt, add a pipe, and make a side blast forge that will burn charcoal or coal, and melt steel.
Many a knife and sword has been made with nothing but a hole in the ground and charcoal long before this generation came about.

My point is, forges don't have to be complicated affairs. I understand you'd probably prefer gas as it's easy, convenient and cleaner.

Anyway, IIRC didn't you build one of Rays forced air burners? With the right forge design/dimensions and a well tuned burner, I believe you should be able to get to welding temperatures without even forced air, but just with an atmospheric burner. There are T-burner plans, heck all sorts of plans, nearly all of them very simple (and cheap) to build.

But I agree with Jim, just build it. The money saved will buy a lot of steel to experiment and learn to do the damascus. Even with the right equipment, you're likely to ruin some billets along the way.