I think the most commonly used guides are the square galvanized tubing, bent close to 90°, and clamped to the main trailer beam, which the PVC pipe fits over. There are two common sizes, with the taller one giving you a better alignment strength, with a taller boat such as a Grady.
Also, if you do not have a crossmember at the very rear of your trailer, you want to mount the guide as close to the crossmember behind the axles as possible. If it has a couple of feet or more to it, that unsupported I beam, sticking out behind the last crossmember has a surprising amount of twist to it, when you pull sideways on the trailer guide.
How your trailer rides may also dictate how much space you leave between the guide and the boat. I have passed trailers on the highway, where the guide was just beating the hell out of the rub rail, because of vibration in the trailer. Mine are pretty close, but if I am going any distance at highway speed, I usually snug them to the rub rails with a dock line across the boat.