The cover should wrap over/around the back seat just fine. I had a saggy spot that collected water and had to put a wooden brace on the seat to hold up the cover to promote drainage. The cover was very durable and held up to 6-8" snow loads over winter even.
Still winterized, haven't tried it, but seems to fit nice and tight. Couple wraps of teflon tape would probably fix it if it did leak. Maybe Wild Blue can report his findings? Too cold up here to try it and not in the water at the moment.
Go to Lowes/Menards/Home Depot and look in the plumbing dept. I found a 4" chrome nipple for like $5. I did find the OEM replacement kit on Amazon, was about $80. Screw that.