Homemade DIY solar heating setups are rarely more inexpensive or more efficient than commercially made panels.Check out YouTube... there are several different methods aside from solar mats that are easy builds and relatively cheap to build and will be a bit more efficient given you have the room around the pool to set up a solar panel... I am building one on a 4x8 sheet of plywood.
I added this Amazon.com : GAME 4512 SolarPro XD1 Solar Pool Heater for Intex Garden to my 16 x 32 rectangular pool. I think it warms it a few degrees.