The Japanese keyboard layout is great for that! I am using Control on the Muhenkan and Henkan buttons left and right of the space bar and Alt on Alt and Katakan

The Japanese keyboard layout is great for that! I am using Control on the Muhenkan and Henkan buttons left and right of the space bar and Alt on Alt and Katakana/Hiragana/Romaji buttons next to them. I also have Caps lock mapped to Control if pushed and Esc when released without another key (xcape) - works great!