Hm...starting to rethink the design. I originally sought after something like the S3 Mini (
link, internal layout is on the page before) but that kind of gives you a really awkward footprint for the case if its laying down flat. It would end up around 300 mm wide and around 180-200 mm deep (it depends on how much tolerance I want to give).
As for height, if I want to keep the case within mITX spec I need to allow 58mm over the CPU area so I would end up with 58mm + 1.5875mm (1/16", standard PCB thickness) + 12.7mm (1/4" standoffs, which has a 1/4" standoff female end and a 1/4" male end adding up to a 1/2" long standoff) = around 72.2875 mm tall or around 2.85" which is already much taller than the S3, even more so the S3 mini. This ends up with a final volume of around 3.9L.
Should I want to ignore that 58mm clearance for maximum ITX component height (it really only matters for embedded motherboards like the ones that come with an Atom or E-350 APU) but still compliant in every other way I can whittle it down to around 2.2" (about 56mm) which is much nicer in size. Using this would give me 3.024L.
Current mockup dimensions with the same amount of tolerance at the 24-pin side of the mobo (10mm) would give me 200 x 180 x 105 mm (a little more than the minimum of 4.1125") which gives me a volume of 3.78L, which puts me close to the volume of the mITX and ATX spec case.
So, any advantage of that design? Well there's really only 2; the ability to store more 2.5" hard drives and a smaller footprint if sitting down flat like a DVD/Blu-Ray player.
Okay, so next up, volume comparisons with a SFX PSU! (63.5mm tall, 125mm wide, 100-130mm long for SFX and SFX-L). I'm picking SFX over ATX because of the fact that 1. SFX is more space efficient, 2. they already have good performing units, 3. not ridiculously tall.
So, for the mITX and ATX compliant case (layout is like the S3), we end up with the same height and length is increased by 125mm. Final volume becomes 6.62L.
For the compliant in every way except maximum component height + a SFX PSU, we have to increase the height to accomodate the SFX PSU so new height is 63mm, final volume is 5.7645L.
And for the current design of "Bento" we end up with an increased length of 125mm, height in the upper compartment goes from 2.2"/55.88mm to 62.5mm~63mm. So we add 7mm in height at most. New dimensions are 200mm x 305mm x 112mm which gives us a volume of 6.832L.