Given the cutting forces involved, I'd say the angle should be sufficient. Even if it does flex, I'd suspect any flex would be less than backlash in the existing nut.
However, if flex does become a problem, provided there is enough room, you could add a couple triangular pieces to brace the angle, using countersunk screws on the underside it can still be bolted onto the cross brace.

Other option would be to get a mount machined from a bit solid bar.
I'd offer to help with any machining, but I'm of on a fortnights holiday in 10days, and already have a queue of jobs I'm needing to do before I go!