Below are some pictures of a 57mm internal height STX configuration with ID Cooling IS-55 on the Deskmini b660. This would be a high performance version.
Tested with a 12600, it sustains 100 watts with temps stable under 70C. This size comes in at about 1.8 liters (as just the case itself, without the frame or any attachments).
The following "universal" modules are designed, so far:
-Drive tray attachment.
Fits two 2.5" drives. Choice of 7mm (pictured) or 15mm internal height. Has adapter bracket for mounting to frame system.
-Case feet for Horizontal orientation.
These re-use the stock case feet pads and elevate the case a bit when it's intended to be used in the horizontal position. The feet are compatible both with or without the drive tray addon. Compatible with threaded/machined aftermarket case feet as well.
-Radiator mounting system.
Hole pattern for 120mm fans and 120/240rads. Can direct-mount to the case, or mount to the frame with adapter bracket. Any orientation possible (tubes up, tubes down, sideways). Alternate lid styles will be designed eventually to support pump/tube passthrough.
STX case should be available soon-ish. Storage, case feet, and VESA are the basic module options for now.