I'm trying to visualize your problem. I've launched many a boat (one year I fished more than 400 hours) but not many times in the last 40 years, since moving to Nebraska. But I still have a little 14-foot Lund with a casting deck I built. I wish I had a video of your situation.
Here's what I think is going on–
- Ramps are in rough shape so you can't get into the water far enough to float the boat off of the trailer
- Consequently, you have to muscle the boat off and on the trailer and you need something slipperier than carpet on the bunks
- You tried velcro because it's plastic and slipperier, but that wasn't totally satisfactory
- You're hoping to find low-friction bunk material
Here are my thoughts, based on the above-listed problem recap—
My friends with airboats on the Platte River cover the bottom of their hulls with Gatorback Airboat Polymer Sheet. You can cut it yourself, heat it and bend it to shape. It's super durable (they drive right up on asphalt). There's a similar product called Wetlander.
I've seen spry-on Teflon in rattle-cans. I don't know if they dry like a Teflon coating or if they're just lubricants for sliding parts, like hinges and locks.