I like it!
As soon my daughter will not need the stroller anymore I will place the spare in the trunk.
Just use the rubber on the sides and top so that the vent is free...

 

When I bought my H3 it had a spare - the wrong size!! I went to a tire store and they were happy to trade my new, wrong sized, tire for a used on of the correct size. They didn't have any in good enough condition for my liking so I kept my spare and went to a different tire place. They also were happy to trade straight across and they did have a decent condition used one. Zero cost, mounted and balanced. I got a used but usable spare, they got a free tire. Even if your spare is used, as long as it's in decent shape I'm sure you could do the same. Worst case, you spend $25 on something. Doesn't have to be new or like new, <motherly voice> but you really should have a spare you can use </motherly voice>.