It ain't pretty, but it works, and it takes all of 30 second to make! I took a pair of Gerber training underwear (any underwear would work, but I liked the added protection of the thick middle),
and sewed a Thirsties doubler/insert (any doubler/insert would do) and sewed it to the outside of the underwear at both ends. I added a few stitches on the sides as well to help hold in addition doublers/inserts that I will slip between the doubler & the underwear.The stitching is hard to see in the pic, so I added the arrows to show where I sewed.
Now I have a customizable protection cloth pull-up that fits my son. I, of course, use a recycled wool cover over that ;)
They have already been, um... tested... and work great! When he no longer wets at night, I can remove the insert with a seam ripper, and will have a pair of underwear & insert again to pass on.