I have done exactly what you wish to do you have a welder so you can too...
What you need is a bottom bracket cup, they have a left hand thread on them,
and a South Paw Freewheel made in India and available off ebay...Here's some pics
how i made mine-->
I had previously cut up a front dual disk brake hub (from ChoppersUS) to make a trike hub disk brake
compatible, i used part of this hub in my left hand freewheel adapter but a disk cut from 3-4mm plate
steel could be substituted just as easily. I only used this because i had it and it saved me time/work.
Simply weld the left hand thread bottom bracket cup to the disk and bolt it onto
the disk brake holes on the rear hub and your good to go...
Best of luck
KiM
p.s the adapter you linked to choppersUS is for a trike axle, it has a right hand thread on it and cannot be used for a left hand freewheel.
I have 3 of those adapters here myself.