How to protect/seal frosted glass spray
Given your time and store restraints, I would agree with your choice of a rattle-can/spray can of matte clear coat.  I don't know the brands well enough to recommend one over another.

There is the potential that the clear coat will reactivate the frosted coats, even if they are thoroughly dried and cured.  So apply very light coats from twice the recommended distance and allow each coat to dry before adding the next.  If the frosted coat does reactivate, it will mix with the clear coat and thus dilute or distort the frost.  The less of the clear coat you apply in each protective coat, the less chance of ruining your artwork.

It might be wise to frost up another piece of plastic and after it dries, test the clear coat on it before risking your art.

If you had more time, an art supply shop (or Hobby Lobby), and an airbrush, you would have more options.  Createx Airbrush paints has a product called Intercoat  which would be perfect for this situation.  It provides a non-reactivating layer to protect your existing artwork from the solvents in any paint or clear coat which you might apply over them.  Spraying a layer of Intercoat over your frost paint would lock it in place, allowing you to add a matte clear coat without any risk of reactivation.