Being simple plastic you’ll be fine. Obviously clamping it really really tight like its on a battleship would be silly, but tight enough so it doesn’t move wont be any problem.
Pace carbon forks has removable clamps on the legs for canti studs., and while they were the correct diameter, it still required quite a lot of clamping force to stop them moving in use. Securing a simple bracket for a guard will be nowhere near the torque you put on those.
Shown here. Now you can imagine something that holds a brake on it would be pretty damn tight.
*** Can I ask what guard it is ?. Im in the market for something like that myself.