I was looking at cast flagstone bases thinking they might be nice to try out- 4 bases for around $5. But everytime I look at some kind of terrain or accessory for my minis, I always ask myself, "Can I make this myself? And is it cheaper/easier to do myself?"
I have plenty of greenstuff and never for a second thought about customizing my own bases. It's definitely cheaper AND easy.
I've only made a couple flagstone bases using the 30mm round "display" type bases. Take a small amount of greenstuff and spread it inside the inset portion of the base. Then use a needle to engrave the joins between the stones. To help the needle glide easier through the greenstuff, I coat it with a TINY amount of oil (I used olive since I had it sitting around.)
That's it! I have yet to try it on a figure with an integral base (or at least an easily-removed integral base.) For slotta-based figures, I spread the greenstuff, then I force the figure into the slot of the base. Clean off the greenstuff pushed through thr bottom of your base and integrate it back onto the top of your base. Then draw your stones under the figure- DONE!
If you have a needle, an extra base and some spare greenstuff, give this a try.