If you cannot find someone with a 20" planer, I would suggest you find someone locally with a wide drum or belt sander and do as I did with a 22" wide 8/4 slab of Bubinga and use the sander to smooth out the surfaces. My slab was over 9' long and it took two of us to manhandle it. The sander was a dual drum 35" wide sander with 80 grit on the infeed drum and 120 grit on the second drum. While not designed as a thickness device, we sanded this 250# beast to 120 grit after about 20 passes on each side (obviously taking very sparse passes each time). I took this picture while my woodworking colleague was feeding this pass.