I havent seen a board ever on a Sprint boot .Mainly because the spare wheel supports the carpet but i am guesssing that you could use one ut it will be a pain to remove in case you will want to use the spare tire.
I installed an 12mm mdf , shaped to the floor of the boot in George's car, so I could install his subwoofer box for the competition.
That was 5 years ago, but he still has it in.
Good soundproof as well
The friendly Alfa Romeo Club - If you are looking to buy or you are already an Alfa Romeo owner, join alfaowner.com today to get the most from your ownership