The GEK building scenario let's you decide the relative amount of "effort vs cost" you want to invest towards your finished unit. The 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 likely give you all the greasy tanks you need.

For the "build it from junk" route, you will need scrap tanks of 10", 12" and 14.75" diameters. 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.)

 

Tanks are usually full of smelly explosive gasses.  Unwanted internal combustion of said gasses will complicate your build.  Make sure to remove all bungs possible, and fill the tank with water or an inert gas before you apply any heat.

 

Getting a nice circle around a tank can be done by using a wide flexible strap of some type to draw an accurate line where you want to cut.  You can also make a "roller bed" so you can roll the tank with your cutting tool is held static.

 

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 them to make the vessels.  A piece of angle iron will guide your torch.  Or a ruler will make lines that you can follow with your sawzall or skilsaw.  See here for cut shapes files.

 

You can cut the sheetmetal to make the vessel tubes, flanges and end plates, using a gas torch or plasma cutter. Potentially even a sawzall, but ugh! Ideally, the tool for the task is CNC plasma cutter, which can run off the CAD files provided here. Many "manufacturing on demand" providers offer cnc plasma cutting services, so you could order in perfectly cut sheet metal to get you started, and not spend all your enthusiasm fighting the prep work. ALL Power Labs can also provide readymade sheet metal and plumbing kits. See here for more information about readymade kits.

Hopefully one of the above scenarios will find a good match with your abilities, time and money available. Whichever route you choose to build the GEK, the final unit is the same, and thus ex