Many cases that come with "full" sleds for drives are compatible with both 2.5" and 3.5" mounting in every sled (as apposed to cases that use "rails").. That beng said, there are cheap adapters that will give you 2 X 2.5" SSD mounts in each 3.5" bay. Those are the way to go in terms of maximizing your SSD mounting without any "cheats." (though DarkSable is absolutely correct, there's no reason they absolutely have to be on an official mount. )
The Arc Mini R2 from fractal design has native mounting for up to 8X SSDs (6x sleds and 2 positions behind the motherboard tray). With adapters for the sleds, it could mount 14XSSDs. That's a microATX case...
The Midi (mid tower) version could do 10X SSDs native, or 18 with the doublers.