I've machined a totally flat base at a size of 58.8mm for the Classic Eazytamp (The Original Eazytamp).
This base design has a splay, like the diagram below.
I've tested it with a variety of baskets, and it fits in a range of different style baskets, including VST (which was for 18/22g of coffee), IMS and also some generic ones I've got.
With the biggest of the biggest steps in its base, it works smoothly and easily. To get it really right for your basket, you can also send us your basket and we can fit the base to size.
If you like the look of this kind of base, you can order it on the Flat Base Classic Eazytamp - just make sure you tell us in the notes section of the checkout that you'd like a base with a splay, and the size you need it (e.g. 58.8mm).
Anyone in the world that wants to copy the design of this base, go for it - you have my blessing.