A ready made solution is available.
Simply purchase a "free standing raised toilet seat"
These are sold by many suppliers of mobility and healthcare equipment.
They are intended to be used with a standard type of flush toilet.
They can be used on any firm and level surface, with a bucket under to receive the waste.

https://www.essentialaids.com/bathroom/t...KxoCS9QQAvD_BwE

Link provided to show the TYPE of product to which I refer and not by way pf any specific recommendation.

And yes I know that this is a very old thread, but the points raised are still relevant.