Silverstone LC16B - capable of mounting 6 harkdisks
You may have a very nice htpc case e.g. Silverstone LC16 and it claims that it could mount 6 hard disks inside. Sounds impressive but due to the size of your motherboard, you could only fit in 4 hard disks and leave another 2 slots empty. Big name Gigabyte has some 'slim' mATX motherboards with size just 24.4cm x 21cm (compared with the standard 24.4cm x 24.4cm), probably it would provide more room for your hard disks.
slim GIGABYTE GA-EG41M-US2H
standard GA-MA78GM-UD2H
The above pictures are for comparison only. Gigabyte and other vendors also provide slim mATX or even ATX boards for 780g. Look around you'll find one.