I've put a section of hot water pipe insulation over the top of most of my folders to help in that direction- and I have an m14 that's isn't a baby in the recoil dept with the paratrooper folding stock- the one made by springfield a few years back- it's also the beretta bm59 folder- i'm only 5'8 and a 30 inch arm- not a big fellow and I can fit with the pad- maybe a thinner outer
Well if one had the Machinist drawings or could make some Machinist drawings for that Topfolder one could then "modify" the design to be shorter and take a recoil pad. Personally as far as Topfolding stocks go the best I've found comfort wise is the Choate single tube design. That has enough room that you can pad the tube.
Remington made a 20 bore 870 on the 12ga frame. They're not easy to find, but they're out there. That folding stock will fit that gun. As for shortening, it's pretty straight forward. Cut a section out of the stock and weld it back together. It's not rocket science but it's not cheap and you need a decent man on a welder. But it can be done with pretty much any welder...I could do it with a stick welder and a 7018 rod.
Scrap the stock! Get a Mesa tactical leo adapter with crosshair recoil reducing hydraulic buffer tube. Put you a magpul moe, ctr, or str, stock on it with the Limbsaver precision fit butt pads they make for em! Lastly shoot reduced recoil buck. Very manageable after that!