A very long strip combined with smelters that you can stack as many as you need. Calibrated to output 2 full belts.
Inspired by this setup https://www.reddit.com/r/factorio/comments/7vey57/belt_based_green_circuit_outpost_with_integrated/
which looks strangely exactly like my own previous build but is actually better.
You can also check <a href="https://factorioprints.com/view/-LRqp9pTAG9DICY3rVNh">Blue</a>, <a href="https://factorioprints.com/view/-LNo9ve36wkEgxNBV9m7">Red</a>