To clarify the question of using plywood. The reason I would be cautious using plywood gernerally is that there are so many types available. To reiterate, all wood is
is a living material, this applies equally whether it is in single plank, block or ply form.

I agree that generally ply is less prone to movement than most woods particularly the softer grades. Nevertheless, there are a number of ply woods that would be most unsuitable. These may be those plys made from any of the resinous wood types. It is also necessary to consider the glue used for the binding.

It is just as important to know whether the ply is a 'round cut' or 'plank cut' Avoid the ply made from round cut veneers as they will tend to crack when any coating is applied to their surfaces.

The better quality plywoods will be 'plank cut' and these are likely to be the marine grades which are most suitable - if ply is to be selected over other materials.

If only a stand by panel is required then a good quality dense hard board will be more than up to the job and will not move at all, it is also very easy to work. Even so when coating ply or board be sure to give both sides equal coverage of sealer or coating.
Stan. L-B