This is about the simplest and most effective, IMHO, trailer keel guide you will find. I use this system on my swing keel VN23 CHIQUITA and it has never let me down in over 40 years. I used heavy wall conduit on Chiquita's trailer, bolted to the keel roller bracket at the bottom and lag bolted to the under side of the bunk boards at the top. I am experimenting with PVC pipe on Monita. I bent it using a heat gun. Next version will have smoother bends. Maybe.

(edit 6/20: It's been 5 years and still working great. I don't think I'll bother fixing the little kinks.)

In a strong cross wind, I let the keel down a little before approaching the trailer. The vertical V shape of the guide lets it slide right down into place. Perfect landing every time, no matter what.

Just my opinion but I think this is more effective than horizontal boards or pipe. It seems to me that with those, unless the keel boards are sharply angled vertically, the keel could easily float over the top of the guide because of the angle of the ramp. Then, when the boat/trailer is pulled up the ramp the boat's keel could settle down on top of the guide. With the vertical V, the boat is automatically centered as the combo is pulled up the ramp even if it was badly misaligned initially.

Update 6/20:

The V guide centers the keel on the trailer when the rig is pulled up the ramp but retrieving is even easier with PVC trailer guides that stick above the water when the trailer is completely submerged. These give an indication of the trailer's exact position underwater. Unfortunately, the widest part of the boat is located right next to the fenders of my trailer. This makes off the shelf bolt-on brackets problematic.
I had a welder friend make a pair of square sockets that bolt to the trailer frame just ahead of the axle. He bent some 1" conduit to clear the fenders and welded a square plug to the bases of the conduit to fit in the sockets. This makes them easily removable. I painted the conduit and slipped a length of painted PVC pipe over each.
They are a little tall in the pictures and I will probably cut them shorter eventually. I will mark the appropriate height when I retrieve the boat this fall. I want to be sure they at least reach the boat's rubrail when the trailer is fully submerged with the boat floating above it.

Update 10/2020
I had initially planned on the goal posts being just visual guides but they turned out to be much more helpful than that. They effectively kept the boat centered while retrieving single handedly in a moderately strong cross wind.