You actually don't have to worry a whole lot about replacing the tank with something bigger - they spend most of their time asleep in the tightest, tiniest spot they can find and fit into, so a 20L is just fine, especially if you take it out several times a week.
What I do, since I also have a screen top on my enclosure, is I mix bark with forest bedding, such as eco earth, in about a 50/50 mix. The forest bedding I know can get too wet (scale rot or R.I.), but the bark also tends to be too dry (your current problem), and yet the two combined are actually pretty close to perfect
Also try covering up one side with a towel, and when your snake is about to shed, just dampening the towel some. That'll bump up the humidity without you having to mist and soak the substrate.
Hope that helps!
~jenny
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1.2 normal ball pythons (Cindy, Darwin, and Periscope)
0.0.1 california kingsnake (tetris)
1.0 rex rat (Scurvy)
0.1 bunny (Spazz)
1.1 betta fishes (Vicious and Killer)
2.2 great danes (Shasta, Odysseus, Merlot, and Watson)
1.0 fat fuzzy mutt (Smokey)
1.1 cats (Thidwick and Turtle)
2.0 horses (Buddy and Sam)
1.0 goat (Billy Jack)
1.25 chickens (Ugly the rooster and his harem)
"God is a comedian playing to an audience too afraid to laugh." - Voltaire