16x 10mm wood screws or slightly longer bolts + nuts to attach the wheels
This cost me about 60 euros.
Instructions for KNAGGLIG rolling laundry cart / storage box:
1. Assemble the KNAGGLIG crates according to the instructions.
2. Attach two crates side by side using a total of four short wood screws that go through the topmost and the lowest piece of wood. Make sure alignment is good. The crate might be slightly crooked.
3. Repeat for the other two crates.
4. Stack the crates and attach the hinges. One in the middle and one in each end.
5. Attach the wheels to the bottom.
6. Make sure any screws didn’t poke through the wood on the insides of the crates. If they did, cut and file off the excess.
Note that my cart doesn’t have a frame. You don’t want anyone sitting on it. It might not be a good idea to store bowling balls in it either. However, it’s very easy to add a frame to the bottom if you need a stronger cart.
With this design, you also need to lift whatever is in the topmost crates in order to access the lower ones. Works great for sorting laundry or other light items.