Tissue box insert

You'll need:
- 7x M2 screws for the rollers
- 8x M4 screws for assembling the frame
- Braided fishing line or some other thin thread
- A weak tension spring, length to be determined experimentally. I used a stainless steel spring, 0.4 mm wire dimeter, 3.5 mm outer dimeter.
- Oil to lubricate the rollers

You don't need any threaded inserts. All the screws screw straight into the plastic. The roller holes need to be drilled to size with a 2 mm drill bit.

The blue dashed line is a tension spring. The side I'm showing is the more complicated side. The wires in orange and magenta are routed to the opposite side, where there are two rollers to turn each of them downwards to support the corners on that side.

I already had a certain length of that spring left from a longer coil, and my intuition was that it had a suitable amount of stiffness so that there exists a length of it that would provide the right amount of force and fits in the frame I printed. Specifically, in the spring formula f