What are your reasons for wanting transparent caves? Is it purely so you can actually still see your fish even though they're hiding most of the time or is there some other reason?
I think we've all had fish that annoyingly hide all the time, damn, i'm going through some current issues with my clown loach which i'm in the process of trying to rectify.
Fish usually hide in rocks or pipes or flowerpots, or whatever you provide them with for a reason. They may be shy, they may be getting bullied, they may be skittish and dislike shadows and general movement outside their tank or they may be nocturnal. Dark, enclosed hiding places can offer them a stress free environment to deal with these issues.
IMO the solution to making sure you can see these fish all the time is NOT to turn their secure bolt holes into see through viewing areas. I think it could be potentially very stressful for them.