I just completed my first project on my 'new' 2013 TC (5000 original miles). I need my window-side arm positioned much better than new vehicle arm rests allow.
I use minicel foam typically used in kayak outfitting. When necessary to glue two pieces together, I use DAP Weldwood contact cement. The 2-inch Velcro has 'industrial strength' peel-and-stick adhesive; it comes on rolls at Lowes or Home Depot.
Even if you had a large enough block of foam to carve in one piece, it's best to build it up, as that simplifies getting the major contours and angles correct.
I use a band saw to cut the foam blocks. Other methods could work, but, the band saw keeps it tidy looking.
Even though the Velcro has adhesive, I use DAP Weldwood to fix the coarse Velcro to the foam. After cleaning the door panel with alcohol, the fuzzy Velcro is attached using its peel-and-stick.
Make sure there's room at the rear of the arm rest, for the shoulder belt to easily clear. If you make the top or side pad too wide, it just takes another run through the band saw.
Here are photos of the finished arm rest. Maybe too utilitarian in appearance for some? The foam can be sanded, so, someone may wish to try sanding all contours fully smooth, and applying an outer layer of neoprene... too much fuss for me.
You can imagine that -- with 3 pieces of Velcro and different orientations -- getting a piece like this in place could be tricky. Just use pieces of office paper taped over the vertical pieces of Velcro, on the door panel. Lower the arm rest in place, to the horizontal Velcro, then slip out the office paper, to fully affix.
Here's one more, of a similar arm rest I made for my current generation full-size Transit. After 5 years, this one did come off (the 'industrial strength' adhesive holding to the door panel gave out). I just cleaned up the door panel with alcohol, and applied new Velcro.