The focal length of a sphere is about equal to its radius. I'd guess your epoxy drop has a focal length of a few mm.

If your camera has a pinhole-to-film distance of 100mm then a 10 diopter close-up lens is about right. The lens out of an old box camera might have a convenient focal length. You might find coverage isn't as great, though, as at large angles a meniscus lens will produce lots of aberrations.