Hello! We never did get our F1 harness trained at all. He could get out of anything (assuming you even got it on him). When we got our first little F3 girl, we took a different approach. We started with ferret vests (thin little nylon ones from the pet store, they weigh nothing). They are not very sturdy and you need to replace the ring on the back with a key ring or something because it is not solid and can come off. But compared to the Hyendry or even pet store vests, these weigh so little that she did not seem to notice it much at all.
We started by giving a treat, putting on the vest, and then giving a treat. We just let her walk around in it for the first several days, if she would start tearing at it or fussing, we distracted her with a feather wand. When we take it off, we give her another treat. Once she outgrew the first ferret vest, we switched to the Hyendry and it was totally fine.
After that, we still repeated this process every day or every other day, and took her on short trips in the car, or on social visits to the pet store or to get ice cream.
She (Siada, our now-11-month old F3) is an angel about it. She sits to get her harness on and then walks right in her crate (usually she gets dressed for regular travel).
We got a second F3 in November who would have been in a tight race with my F1 for who could be a bigger butthead, but once I can nab her, she still gets dressed with no complaint. (We did the same with her with treats putting it on and off.)
Toying with the idea of getting an F2 boy, but worried I might break our good streak with cats getting dressed. LOL!
Wish you luck!