Here is my journey building my propane forge. I’m not looking at this as a coal forge replacement,  but an addition to my blacksmithing tool set.
This forge is a compilation of watching a whole bunch of YouTube videos, reading blogs and websites. You don’t typically find two the same, so it seems it’s best to pick a design and go with it.

http://www.zoellerforge.com/
https://www.instructables.com/id/How-to-Make-a-Propane-Forge/
https://www.youtube.com/watch?v=hqFDRBZZPWQ
https://www.youtube.com/watch?v=thNOSEhcQog
https://www.youtube.com/watch?v=_bnMj4TjKjk

zoellerforge.com was especially helpful and I wound up ordering everything I couldn’t find local from there.
Here is a list of what I used:
A 7 gallon  air tank (an old tank that’s been around forever)
For the burners
(2) 3/4″ black iron pipe tee
(2) 3/4″ x 8″ black iron pipe
(2) 3/4″ to 1″ adapter (this is because I couldn’t find a 3/4″ x 1 1/4″ adapter)
(2) 1″ to 1 1/4″ adapter (flare)
(2) 1/4″ plug (drilled and tapped with 1/4″ x 28 for nozzle)
(2) .035 nozzle for mig welder.
(2) The ball valve came with the connection kit from Zoeller Forge
Here was what I bought from Zoeller Forge (for 2 burners)
(1) Two burner connection kit
(4) 9″ x 4 1/2″ x 3/4″  3000°F heavy duty fire bricks  (2 extra for when I use flux to forge weld)
(2) 2 1/2″ x 9″ x 4 1/2″  2600°F insulated firebricks
4 lbs Plistix 900F
(2 each) Propane Quick Disconnect and Coupler
(4 running feet) 1″  8# density 2300°F Durablanket 24″ wide
Building the burner.
This was pretty simple once I figured it out. I drilled a 1/2″ hole in the top of the tee. With just a little filling, the plug fit through the hole. Next time I think I will tap it for the plug.
I drilled and tapped the plug 1/4″ x 28 to accept the .035 nozzle. The plug fits through the hole and the ball valve threads on it to hole it in place nice and tight.
The propane connections added per Larry’s instructions.
Building the forge body
I cut the front of the tank off following the original weld line. This is just to facilitate the ability to work inside it. This cut was made with a 4 1/2″ grinder with a cut off wheel.
I used a 2″ hole saw to cut the holes for the burners.  You can obviously use a different technique  like drilling a series of holes around and knocking it out and grinding or filing it round.
The black pipe adapters were welded to the tank
I just made a few brackets to bolt the front back on.
I have seen a few designs where the door was hinged. The front opening on mine isn’t much smaller than the size of the box, so I didn’t see an advantage
I made the back opening the size of the insulated fire brick. This allows me to slide the brick in to reduce the box size and use a single burner.
The shelves on the front and back are just 1/8″ plate steel.
The front opening  is also the height of the firebrick, but wider. I can use the brick as a door to  close down the opening when appropriate.

MY PROPANE FORGE BUILD 001

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Here is the back view. The firebrick is just closing the door. This allows me to slide the brick in to reduce the box size and use a single burner.

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The firebrick can be slid to open or close the front opening

MY PROPANE FORGE BUILD 004

Here is the 3/4″ to 1″ adapter (this is because I couldn’t find a 3/4″ x 1 1/4″ adapter) then the 1″ to 1 1/4″ adapter to create the flare

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MY PROPANE FORGE BUILD Parts

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MY PROPANE FORGE BUILD Valve

A simple bracket to hold the front back on. The tank is threaded to accept the 1/4″ x 20 bolts cut to length.

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I had trouble getting one burner to work correctly. It wound up being the nozzle was partially plugged. After figuring it out, I replaced the nozzle and everything was good.