Put a thicker layer of cypress mulch. The top dries quickly, but the bottom usually stays moist. In my rainbow boa enclosures, I use about 1.5" of cypress mulch(pile it in...then smash it down). I mist it slightly once every other day. If the humidity in the enclosure is insufficient, they will go into the mulch and utilize the moisture there.

However, you are keeping ball pythons...not rainbow boas. Since rainbow boas can tolerate a higher substrate 'dampness', they do not have a high risk getting scale rot from this practice. I can not say the same for ball pythons, so be careful with making the substrate too damp.

What is your humidity level without the mulch?