IMHO I wouldn't add much more bracing than is already called for on the plans. The enclosures are pretty small and should be pretty resonance free with minimal bracing. That should help a bit in fitting the crossovers. Plus adding extra material to the box without taking it's volume into consideration might have a negative effect on the F3 point.
Also I would ditch the plastic speaker terminals. Nothing like building a solid enclosure then cutting a hole in it and inserting 1/8" plastic. Binding posts directly in the MDF or using some aluminum plates with minimal holes cut out would be a better route. There some nice Dayton ones at Parts Express which are not that expensive.