You can try placing a secondary water dish on the warm side. Pick a flat type of dish, non-tippable but with a larger surface area. The larger terracotta plant pot saucers can work great for this. This being placed on the warm side should allow for more evaporation and help somewhat with your humidity issues (it won't add much though but some).

You can also try substrates that tend to hold humidity in better. Double or triple milled cypress might be a good option in your glass enclosure.