Heating an enclosure

I'll be visiting some local hardware stores today to start getting some prices and advice on what I need to build an 8x4x5 enclosure. I want to have 3 levels with ramps leading to the top and a basking lamp on the highest level. However, I'm not sure how to maintain a hot and a cold side on an enclosure this big. Anyone have any ideas?

i have a heat pad (10"x11" heat tape) under some tank carpet under the sand where the hide spot it the i have a 160w uva/b an heat reptisun bulb i believe it is... works great top stays right near 103deg bottom is 80s-90s