The GEK building scenario let's you decide the relative amount of "effort vs cost" you want to invest towards your finished unit.  As basic vessel dimensions are based on common scrap tanks found in North America, so you can choose to build it for minimum money (and maximum effort) with the dimensions, instructions and CAD files provided here.  The local junkyard will provide all the greasy tanks you need.

 

The "build it from junk" route requires scrap tanks of 10", 12" and 14.75" diameter. 10" is typical for hand held air transfer tanks and some truck pony tanks. 12" is typical for 5 and 10 gal propane tanks. 14.75" is typical for a 100lb/25gal propane tank.  Warning: There's a surprising amount dimensional variation on "standardized tanks" between different tank manufacturers. This can complicate the fit of flanges and end plates to the scrap tanks.

 

 

Scrap tanks are usually full of smelly explosive gasses.  Unwanted internal combustion of such gasses will complicate your build.  Make sure to remove all bungs you can wrestle off and fill the tank with water or an inert gas before you start cutting.

 

Marking an accurate circle around the tank can be a challenge.  Try to find some sort of wide stiff strap to wrap around the tank to use as a guide.  A thin metal strap is usually the best, something like wide metal plumbers tape.  You can also make a "roller bed" and roll the tank against a static marking object.

 

Your options for the cut are a cutoff of wheel on an angle grinder, a cutting torch, or a sawzall.  All will work, so you decide the tool you are most comfotable with.

  

 

If you have a roller, or access to a roller, you will save yourself a ton of hassle if you just purpose cut 16awg sheet steel and roll the pieces into cylinders to make the vessels.  This makes it easy to get exactly the correct diameter to fit inside or around the flange rings of end plates.  "Predetermined diameter" cylinders never fit the flanges and end plates exactly as you want them to.

 

Cutting rectangular sheet metal pieces is easy.  Use a piece of angle iron will guide your torch on a nice flat surface.  Or use a ruler to make lines that you can follow with your sawzall or skilsaw.  See here for cut shapes files.   Scrap sheet metal is near everywhere.  Or buying it new is not terribly expensive.