There is a type which has a cylindrical chamber which is just the right height and diameter to adequately cover a standard 76mm x 26mm glass slide. The technique was to put the slide in the funnel and cover with the plastic solution. Let it settle to avoid any lumps or bubbles and then turn the glass valve at the bottom and drain the plastic into its stock bottle or a beaker. You can even get thicker layers by pulling the tap out to rapidly drain off the plastic. Of course, do this in a fume hood.
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I did serial sectioning for years on large single hole grids using a very simple technique which made the problem of film thickness and wrinkles a very minor one. Even though this method does not deal directly with ways of determining film thickness, it may help you out with the specific problem at hand. At one end, drill about a dozen holes, roughly 5mm in diameter in an area about the size of a formvar film cast on glass slides. These slides will serve as your template for holding your films. Usually a good silver film, not grey, will work fine.