I have an SEL and a SHO PP. I believe the SEL's 17" spare (-11% diameter difference vs the stock 245/45 20) was a little too small for the SHO's AWD system, compared to the 18" spare.
So, I went with an 18" alloy spare from an Explorer on Ebay, P/N FB53-1007-TA, with a T165/70 D18 tire, with a closer diameter to the stock 20s (-5.6% diameter difference). I've never used it, but installed it on my PP's rear wheel location to test fitment/brake clearance without issue. (See my pics)
Don't forget, to complete the OEM-looking install, you will need a jack/lug wrench kit AND the jack retaining bracket and nut. Some kits include the bracket and nut and others do not.
FWIW, the Flat/Inflator kit that came with the PP has a sticker on it that says not to exceed 50 mph. So technically, both options are only good for 50 mph.
While most functional as a spare, the full size 18" steel spare option is more pricey and will reduce trunk storage. Here is a post including the P/Ns for the foam insert and matching trunk cover:
For those looking for part numbers for the PI foam surround support and cover, I found the part numbers. DG1Z-5445026-AA for the cover, and DG1Z-9913546-A for the foam box assembly. What I found is they are both very pricey. Over $700 for both pieces. At this point, I decided to make my foam...
shoforum.com
For PI vehicles, the full size 18" steel wheel installed in the factory location is required for their 75 mph rear-end impact rating.