I tried to make my own gradient filament but the ones I've found were meant for beds smaller or larger than mine. I guess others might have the same issues.

Here's a 250mm diameter spiral.

Process:

1. Set a low layer height: 0.1mm
2. Set z hop to 2mm.
3. Perimeters only.
4. Set your purge line to be somewhere safe, e.g. off the bed.
5. Print the bottom part.
6. Print the top part.
7. DONE!