Here's the set of inserts I created for the 75mm bases I previously uploaded.

https://www.thingiverse.com/thing:2909709

I focused on the 75mm size because the smaller bases tended to be mostly filled by the pre-cast bases from Reaper Bones minis, with the small gaps more easily filled in with normal basing methods.

For best results, use these as the "base" of your base... adding flourishes such as static grass, dirt, snow etc to break up the monotony.

These were made using TinkerCAD and OpenCAD.