Using your Ikea case idea the approximate size is 30 cm EDIT: by 40 cm. So approx. 7 rack units, plus a Wilba/TK SEQ is 10. That's quite a bit of real estate, and I'm wondering whether the SEQ belongs in the rack (MIDI'd to your synths) and the BLM on a keyboard stand/ table/ hand-held as a real performance instrument.
Plus, it's gonna be battery powered and WLAN MIDI'd right? ;)
I'd say people will have a lot of opinions on which buttons to use, my humble suggestion is to keep the layout as general as possible so peeps can choose. SPST tact switches usually have 200 mil pin spacing, correct?