a toilet's stability

I do the same. I put a chalk mark + on the floor so my aim is close

adjustments.

before the days of wax rings they used a plaster like mix of newspaper and plaster or something like that and it was called frit i think. maybe im just forgetting the real name but i thaught it was frit.

Plaster of Paris & newspaper clippings was used to make "dynamite". That was how we (plumbers) capped off risers (baldies and hubs) to perform tests. It may have been known in other areas by other names.

Bob Wheatley

Try building a substantial wooden box around the toilet and mount a toilet seat onto that surface (over a hole of course). Surround the alcove/box with some heavy duty grab bars and you have something. The trick is to not let the fat guy actually sit on the toilet ( I imagine an outhouse bench over a real toilet). Make sure to hinge the top and front so it can be cleaned. He can even put a carved wooden backrest behind it and call it a throne.