I still don't understand boat's not coming with a spare, EVERYONE needs a spare, I know its probably a profit item, and would add a few $$ to the cost of a new boat, but I think its silly! My new 1999 Tracker didn't come with one, didn't realize that till I took delivery (my first new boat), and had to pay for the tire/rim and the mount itself. My current boat didn't have a spare when the dealer got it, I negotiated a spare in the deal, and when I picked it up realized (dealer and I) that the trailer (1999 Astro) didn't have a built in location for the spare, so I just ordered one off Amazon and will install it this weekend.
So I think looking at your Z8 it has the location right up front on the trailer so you just need the tire/rim, and bolts/lock.
Great looking boat, you'll enjoy, and safe trip back from the Rig!
Trep