Yeah, 20 count humidors are pretty much a waste of time except for the odd stick or two and maybe better serve as a repository for spare change, lighters, cutters and all. The dollar bill test works pretty good...shut the lid on a bill and tug it out. It should have a good resistance if the seal is tight.
One trick you might use if you want to to keep the humidor in service is to build up the lip with scotch tape. That will work after a fashion and tighten the seal up a bit.
It's fairly common practice for the cheaper humidors to be made with an MDF body covered with a thin sheet of wood veneer. That usually lends for a fairly stable humidor if the veneer covering the MDF is well applied with a water resistant glue and the overall construction of the box is up to par. I'm always suspicious of Spanish cedar veneer on the inside because it doesn't have enough mass to hold moisture all that well. I far prefer the Rubbermaid containers with sheets of cedar inside, or, even better, my own homebuilt humidors to hold moisture.
To each his own though.