Google "bottle bed tray hack" under Images and there is a video that seems to use platiic pipe rather than bottles for the legs. The benefit is that a water bottle fits in one of the pipes.

If you can get a piece of stiff cardboard big enough, you should be able to adapt the pipe version to fit bottles instead. Trial and error. The cardboard used is pretty thick, not a standard box.

A question - does the nursing home not have bed trays or roll across bed tables for the patients to use?

Would building your bed tray using PVC or ABS drain pipe, but leaving the upright sections not glued for easy on and off assembly be a solution?

What I imagine is taking 20 oz bottles, one for each corner. Turn them upside down and mark them at the point (height) you want the table top. Cut a slit long enough for a corner of the top to slide in all the way, and just wide enough top to bottom to accept the top but hold it firmly (or, instead of making the slit wider, clip the upper edge slightly at each end and perhaps in the middle just to allow enough flexion to hold the top.

However, I am not sure that the bottles and cardboard would be sturdy enough for practical use.

Thanks all! It would be understandable to me in the form of pictorial instructions. I actually meant to say layers of corrugated-carton cut-outs (rather than cardboard). btw, I can't easily get hold of pipes, nor cut them to size. The nursing home has rolling tables but her particular bed doesn't enable the table to roll under it. Other beds at the home don't have that issue.

Could be they specifically assigned her such a bed due to her sometimes falling when trying to get out of bed. That type of bed may be designed with a mechanism that lets it descend nearly to floor level (on one of my visits her bed was positioned nearly at floor level).

OK, now, please consider this thread wrapped up, as it's probably impossible to achieve. Thanks again.