From my experience on my UDS the bottom stays pretty cool since all the heat is going up and out. The bottom of my charcoal basket is about 3" from the bottom of the UDS. I have an ash pan and about 1" "legs" out of the bottom of the pan that sit directly on the bottom of the UDS.
I can easily place my hand on the underside of the UDS and not get burned.
If it does get too hot and start to melt, there is a company that makes a soldering mat (for a lack of better terms) to prevent heat from catching unwanted things on fire. My dad has one and showed me the demo he got from the salesman...melted a nickel with a handheld propane torch while he held it in his hand. You could cut it into strips and place them between the bottom of your UDS and the PVC.
Homemade trailer Roaster a.k.a "Hog-A-Nator"; UDS; Dual Chamber Charcoal; ECO Gasser