I would argue the following. A mezuzua case needs to be constructed so that the halachot of hanging a mezuza can be followed.
- The casing should have a clear "top", so that the mezuzah can be hung appropriately (not upside down).
- The casing should be taller than it is wide.
- The casing should have an opening large enough for a scroll, so that the scroll can be rolled, and not bent or shoved.
- The casing should not look like some other religion's or religious symbol that is often hung on doors or walls, so that it is clear that it is a mezuzah. (For example, I don't think you could make it into a cross, or horseshoe, or a standard Chamtza)
- The casing needs to be weatherproof so that there is no danger of the scroll getting damaged.
Regarding opacity. In Israel they have a few "standard" mezuza cases, these cases are all the same shape and come in white, black, brown, silver.. AND transparent. So I don't see what is wrong with a transparent mezuza as hudreds of people have them.