I agree with Maui on getting the H type harness. The figure 8 ones seem too flimsy.
Carefully test the tightness of the harness. I was able to test mine by putting it on the cat, then holding on to it as kitty tried to back out of it
. I found that if the harness wasn't tight enough, he could just come out of the whole thing! You want to be sure that your cat won't be able to escape if he tries to pull away if he is startled by something.
For cats that don't take well to collars and harnesses, I find it useful to put the harness on for a little bit each day to let the cat get used to it. Some cats don't mind, but others act like you've attached a 10 lb weight to them