You will want to get that tank off the floor, or at least have it raised a bit somehow so there is proper air flow for the UTH - it is a firehazard otherwise!!
I love the idea of carboard hides. They are great when snakes aren't done growing! Or if you just don't want to invest in super expensive store bought hides!!
When controlled by a thermostat there is no need for an air gap for the UTH. The thermostat will prevent any heat build up from happening.
Cardboard hides are fine but they will need to be changed regularly to prevent bacteria/mold growth.