I agree, these things are a pain in the AstroTurf to get back on!!
When I put the new firewall cover on Frodo, I had to pull everything off the firewall, and I purchased some new ducting as well. If I recall correctly, I put it on the heater box first, getting it all the way on. Then i mounted the box on the firewall. I then got the ducting on the backside of the vent, then used a butter knife with about 1/2" of the tip bent at a 90 degree angle to work it around the top, bottom and front side of the vent.
As I said, that's how I recall doing it. It may be easier to mount it to the vent first, and then use the bent butter knife on the heater box. These things are frustrating, I know. Just keep at it, you'll get it! My thoughts and prayers are with you!
