The emergency toilets that I have seen consist of a folding frame similar to a folding chair, but with a toilet seat under which is placed either a strong plastic bag, or a bucket.

Much easier to use than squatting especially for the less fit or those unused to having to improvise.

These light weight folding articles are indeed unsuitable for those over about 250 pounds or 110 kilos. Different makes vary a bit.

I have prepared as follows at little expense and with minimal space taken up.
I purchased a piece of heavy duty plywood about 30 inches by 45 inches, in the middle of this I cut a toilet sized hole.
Over this hole I fixed a standard plastic toilet seat complete with hinged lid.
The two longer sides of the plywood are reinforced with square timber screwed to the under sides.
When not in use this contrivance may stored on edge behind crates of other supplies and thereby take up little space.
When required for use, the board should be supported at each end on strong stacking crates containing doom supplies, choose crates to give a comfortable sitting height.
Under the seat I would place either a bucket containing cat litter or other absorbent material, or a bucket lined with a strong garbage bag. Ensure that only a minimal gap exists between the rim of the bucket and the underside of the plywood.

The plastic seat is readily cleaned if it becomes soiled.
The level surface to left and right of the seat would be handy for placing a toilet roll and a flashlight or lantern.

The supporting crates may still be utilised for storage, I would not much fancy keeping food or drink thus, but toilet paper would be fine.

The cost was about £25, excluding the strong crates that I needed anyway.