The requirements for such a medium are that it won't deposit crystals when it evaporates and that it won't be incapable of evaporating. The second condition probably excludes anything containing glycerol. The first is more difficult. A polymer such as gum, polyvinylpyrollidone or gelatin might fiil the bill, especially in the presence of a sugar that can form a syrup that1 s almost solid. Fructose (levulose) has this reputation, and is a good aqueous mountant. Very cheap, but for some reason somewhat too acidic for basic dyes, but can be buffered. Its not 100% predictable. l've had fructose crystallize under coverslips, but you can quickly remove ail the mess with water and then try again.

An old and certain way to make a solid and perhaps permanent aqueous mount is to use a smaller-than-usual coverslip and as little aqueous mountant as possible.