Another possible fix: spacers for the granny mounting bolts. You won't need much--maybe 0.5 mm or 1.0 mm should do the trick. These would be cheaper and a lot less trouble (and a whole lot lighter) than that Frame Guard thing.
Assuming your fder is correctly positioned on the seat tube (proper height and angle), do the following:
1. Shift to the lowest gear (granny chring, biggest cog).
2. Disconnect the fder cable.
3. Adjust the low limit screw so that the the cage is JUST clearing the chain plates. Make sure your chain clears the rear tire.
4. Reconnect the fder cable.
5. Shift to the highest gear (big chring, smallest cog).
6. Adjust the high limit screw so that the cage is JUST clearing the chain plates.
7. Play around with the drivetrain. Check fder shifting and cage rubbing in different conditions, especially when cross-chaining. (I mean, nobody ever crosschains, and I know YOU never would, but just in case, see how the fder behaves anyway.)
- If it rubs against the middle chring when in the granny gear and crosschained a fair amount (say, to the 3rd-smallest or 2nd-smallest cog) consider spacers for the granny. Again, thin ones (0.5 or 1.0 mm) should do the trick. This will ease front downshifts anyway, and will likely prevent even occasional overshifts onto the bottom bracket shell.
The other thing to consider: how often are you really in your lowest gear? If it's not very often, a TINY bit of chain rubbing in the lowest gear (between the chain and the inner plate of the fder) may be OK, especially if the alternative is overshifting during a long climb.