A way to make rollers for spools at a specific size and curve.
The default design is a 89mm long spool that has a 8x6mm pin that go into a 608 bearing, a 11.5x0.2mm padding to prevent friction in the larger roller, then a roller 34mm diameter going to 30mm then back to 34mm, another padding and the pin for the other bearing.
You can adjust it to your need. Smaller bearings, larger bearings whatever you want.

I used this to design the perfect sized roller for my (cereal box) dry box. The width my cereal box needs is 91mm. However I used 6mm for the pins that go into the 608 bearing instead of the 7 so I reduced the size by 2mm so 89mm. Be aware of this if you make one for yourself

If you don't want a roller but something else you can use the shaped_roller function.
If you want a curved hollow inside you can do shaped_roller( 30, 25, 50, 35 ); This will generate a hollow tube that is 35mm in diameter and the inside starts with 30mm gets narrower to 25mm and back to 30mm at a length of 50mm.
shaped_roller( 30, 25, 50, 22 ); will make a roller that is hollow on the inside 22mm, outside 30mm at the top then 25mm in the center and 30mm at the bottom, 50mm height.
You can also do shaped_roller( 25, 30, 50, 22 ); which would be 22mm hollow, outside starting at 25mm center 30mm ending 25mm, 50mm height.

I hope you enjoy. If it is slow you can use the nightly OpenSCAD build and use Preferences > Advanced > 3D Rendering > backend > manifold. But from my testing even a high quality output only takes seconds. Feel free to ask questions.