What a fantastic project and result. They are quite durable too, if their numbers are right each roller can go for about 10.000 km which is good for about 3 years of use. I suggest improving the product with 2 rubber belts acting like tires to reduce physical effort needed and improve durability, and to optimize the shape so it doesn’t get scratched by rocks. Additional improvement could be a simple seat over the axle which makes it into a primitive stroller. I don’t see why they have to be so expensive though at $125 each, I am sure the cost can be much reduced.