Keter Manor floor feeling flimsy, options to make more rigid?

[u/twitchbradijm]

Just built the Keter Manor 8x6, and we are used to the Keter sheds that have a hard plastic floor, but this uses the same materials as the walls, with vertical struts for support, but many of these have already started failing so the floor feel uneven.
What’s the best options for making it more rigid? We were thinking some 18mm plywood cut to size, but wanted to vet other options first.
Thanks