My new (used) boat's trailer came with broken guide ons; they were made of 1.5" PVC elbows/pipe that was attached to the trailer frame with large stainless steel hose clamps. Cheap, effeective until they broke off.
I suspect that they broke because PVC is somewhat brittle to start with and it becomes more so with exposure to the sun.
Part of the problem is that you want enough strength to actually guide an off center boat to the center, against the wind, yet to also cushion the blow, to avoid damage to the boat. Some trailers have a pair of long flat angled carpeted fender surfaces while others have cushioned rollers on posts.
Black ABS pipe seems to be stronger than PVC with thicker walls and larger diameters than PVC.
Or, maybe a piece of spring steel, such as a small car's sway bar? I've seen them at less than 1/2" diameter. It could be welded onto the trailer frame and then its vertical section properly cushioned/rollered? They're already bent at something less than 90* Just extend it from the bend to the trailer frame and then up to a roller with steel tubing?
Or, maybe a 1.25" garage door spring, covered with a flexible sleeve to prevent hand pinching, welded at the correct angle to a steel extension welded to the trailer frame rail?