I like that drydock, but I am not much of a builder
I did build my own "lift" today, cost me $115, but I am not sure I like where I went with it.
My goal was to lift it out of the water, which I accomplished, just not sure how sturdy it is and it is allot harder to lift the boat just a few inches out of the water. I am not sure I am allowed to post pictures here as a newbie.
So here is my first attempt at getting my boat out of the water.
I pounded four 3/4" steel pipes into the ground about 3.5 feet (6' long near shore, 7' in deep water) until there was about 4" sticking out of the water. Then I took four 5/8 re-barb L shape (4' x 1'). I wrapped these with a foam noodle and fitted a brass fitting with a washer to set the height. I then put this inside the pipe about half way down. This allows the L shaped end with foam noodle to spin around. It also allows me to adjust the height up and down when the lake goes up and down. I can go down 2' and up maybe another foot.
I take the 2 in the deep water out and pulled the boat over them and up the 2 closer to the shore. Then I put the 2 in the deep water back in. The L shape tops covered with foam spin freely.
Here is the hard part, I then have to lift the boat up about a foot and spin the re-barb under the trampoline and secure them with a tie-down. Then go the back and repeat the process.
I started with the boat of the pontoons being 6 inches out of the water and I just couldn't lift it that high. I now have it about 2-3" out of the water. I will see how it looks in the morning and take some pictures, but I think your solution with a wench might be easier for just one person.
I thought it would be better to attach under the trampoline than the pontoons. With your solution as you stated, there are 2 high pressure points which I don't think would be good.