I have the Triumph tank bag (love it!) but sadly, they are correct when they say there's no room for a tach. I used to use magnetic tank bags and they worked just fine, but I needed a large size and the magnetic ones that were big enough looked kinda lame.
Do you think that the magnetic tank bags that you used in the past would work with a tach? If so, do you recall the manufacturer of the bags that you have tried?