There are tons and tons of kayak crate DIY videos on RU-vid. This is the first time I have seen this moveable axle idea. I love it. I implemented it last night and it works great. I made some minor changes. First, I put a lock nut on one side because I will never be removing that side. Second, I skipped the nut on the other side and just went with the hitch pin and two washers. Finally, I drilled a pinhole through the threaded rod for the hitch pin. It makes the pin much easier to insert and remove. Thank you for this great idea.
Try flipping it around... Aka wheels to the front. You could also drill the hole in the middle of the crate, as opposed to the ends. Bigger wheels might also provide a solution. Try replacement pressure washer wheels.
As far as I understood, he is referring to a Milk Crate that you can purchase at Canadian Tire, Walmart, or Rona. I do believe that depending on the dimensions of the crate, the length of the threaded rod will change.