When I mounted my 21" CRT I did something similar using 3/4" plywood as well, but I used 1x2's glued and screwed on both sides of the cabinet, and then I screwed the CRT-frame into those 1x2's on each side. It has supported the CRT fine when moving the cabinet around the house. Keep in mind that I used a Trinitron-style CRT, so it is very heavy for the size due to all the extra glass in the tube.
The only pain-in-the-ass I had was cutting the hole in the 3/4" plywood for the CRT to fit in - the mounting ears on the CRT didn't stick out very far compared to the tube dimensions, so I ended up cutting a curved-rectangle where each side bowed out slightly from the corners. I wasn't 100% sure about the weight support like you, so I figured having the most wood around the CRT would be best - I ended up with at least 3" per side for support.