I recently built a custom 3x2 stand for my Korg Volcas.
I had a look around on the Net, there are plenty of projects and templates, but none was right for my idea.

I took the template from a template file created by Oliver Dowie that can be downloaded HERE. The one he created is only 2x2 and has big wooden sides in order to keep the line of Volca inclinated.

I decide to go flat and modified his sides, keeping the Volcas flat with the surface they are laid on.
I cut 2 10cm x 58cm stripes of wood (the Volcas are +/- 19 cm. each) for the bottom pieces.
This way I can simply put the three volcas side by side and they fit quite well in the case, once it is screwed.

I used some plywood (about 3 euros at a local store).
I simply glued the parts together and once the glue had dried, I screwed the sides and the bottom plates.

Easy enough

(and just before you ask, that grey DIY "Volca" is just a MeeBlip Micro built into an old LaCie Hard Drive enclosure, which by chance is nearly identical to the Volcas size...)