Place a length of metal wire(weld wire, all thread, music wire etc.) through the tubing marking the surface inside of the circle. Take a heat gun and heat that side of the tubing it will expand and when left to cool it will stay straight. This time of year I just put it on my mantel and let the wood stove do the work. If you don't have a length of wire long enough you can hang a weight on the end and use the heat gun in the same fashion. It's pretty tough plastic but a Monokote gun can overheat it if you stay in one place too long.