Let's Twirl Again ....

This customizer will let you build a twirl pattern of whatever size you like, along with a backing plate to add contrast to the print. The pre-created STLs are for a 120mm square mat of depth 3mm.

The customizer will let you fiddle with the colors to get a feel for some alternate constrasts. You can create a custom thickness, specify the layer height you intend to print with, and the number of base layers you want. I've printed with 3 base layers.
(For some reason, the customizer doesn't want to play ball. Could be due to the size of the scad file. You may need OpenSCAD to modify.)

You can happily stop your print and switch the filament after the base layers have printed, or just let it all print in one go with dual extruders that are nicely calibrated (http://www.thingiverse.com/thing:1480365).