I have a similar problem living in AZ, but I wouldn't recommend using the reptifogger for a 20 gallon. I used one on my leopard tortoises when they were babies, which worked great for a huge 6'x6' enclosure (even then it was on a timer and only came on for 2 minutes 3x a day), but even on the lowest setting would be massive overkill for anything smaller than a really big arboreal cage (or amphibians who need a way more humidity than reptiles). I use the spray bottle method like darthkevin. I do not recommend that hydrometer though, analog ones are notoriously unreliable. May people on this site have had success with the acurite brand weather thermometers that give two temperature readings (warm end, cool end) and humidity. I got mine on Amazon for $13. Wood will be okay for building a cage if you use a sealant on it, but make sure it's one that isn't going to give off fumes or leach chemicals. I would vote for melamine if you really want to build your own cage, it's usually available in stores like Lowes or Home Depot.