Use double-width keys relegendable keys. These came with my programmable Genovation keypad, but I think that they can be bought separately from various places.
Show Image
(http://geekhack.org/attachment.php?attachmentid=14868&stc=1&d=1295218107)
Your photo is just what i'd like to achieve. How stable does the key feel when you hit it at the edge? My concern has been that without a metal bar to distribute the load, the key won't depress evenly. I did an experiment with a long 'shift' key on top of two MY switches. It didn't seem to want to depress evenly.
This:
Show Image
(http://www.personal-media.co.jp/utronkb/images/key-m.jpg)
I use it with the Colemak layout. With Muhenkan as Win (the big key on the left side to the left of the left shift).
Henkan (the other one), is a remap key I use for random stuff.
Hiragana/Katakana (below Henkan) I use for AltGr (Colemak requirement :D).
How much did you pay for it? Where did you get one?