I use a 40 lb propane tank, which I purchased new so it would be empty.
I don't recommend cutting into a used propane tank, since they can explode.
You can fill a used propane take completely with water, and then cut into it, but most don't know how to get them completely full of water.
I am not entirely pleased with how it turned out, but it is functional.
It is tricky to stick weld fittings on a thin-walled propane tank, and in spite of my efforts, and two passes on the welds, they still weep air.
I was wise enough to keep all the fittings at the top of the tank, knowing that any leaks would just be air leaks.
I use 10 psi compressed air on this fuel tank, with diesel, and have a 30 psi safety valve on it, since I heard of one individual who had his regulator fail, and put about 100 psi on his fuel tank and fuel lines.
The copper tube goes to about 1" above the bottom of the tank.
I have seen a better fill cap used by someone else with a similar tank, and it is a 2" ball valve, with a permanent funnel attached, which is made of a 4" PVC plumbing bell.
I will add a valve and funnel to mine when I get time.
I cannot re-weld my welds because one you get diesel in a tank, you don't want to weld on it again.
The air leak around one of the fittings is minor, and so I ignore it.
I use a ball valve for fuel cutoff, and a second ball valve to release the 10 psi on the tank when the melt is complete.
Some folks mount their tank on a 2-wheeler for easy transportation.
You can also place the tank in a milk crate to help prevent tip-overs.
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Edit:
It is a little difficult to see in the photo, but the copper pickup fuel line curls around and connects to a black fuel line, which runs to the burner.
There is a needle and ball valve at the burner, but not at the fuel tank.
The clear line is the air line into the fuel tank.
The ball valve with red handle is the pressure release valve.
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Edit02:
A word to the wise, and I repeat, I would recommend using a new propane tank that has never had gas in it.
I have read more than a few stories posted online about folks doing hazardous stuff trying to re-use a used propane tank.
Here is one story:
A guy says "I opened the valve on the propane tank, and put it out in the middle of a field for two days to drain the tank".
New propane tanks have a safety device that prevents propane from leaving the tank unless a hose is plugged/screwed into the tank.
So this guy's valve did not vent the tank.
He then takes the tank into his shop, and drills into it with a drill, and much to his surprise, the tank was still full, and it vented the propane into his shop in a big cloud.
How he did not blow his shop and himself off the face of the earth, I will never know.
Don't repeat other's mistakes.
Also, the factory-installed valve on the top of a propane tank is not easy to remove.
They are designed to never be unscrewed, and so it was a bit of a battle for me to get the factory fitting off of my tank.
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