The sliding cargo tray option will give you a flat surface, but the divider you linked is just that little vertical piece at the front edge of the sliding cargo tray. The cargo tray is something you have to get separately, and it's not cheap to buy new from the parts department. IIRC, it's like $1600 for all the parts, which is really dumb because the option is only $350 from the factory.

Lots of people hate it, so you may be able to find someone to trade with if it's available in your region. If you're able to configure a 4Runner with the sliding tray on the website using your ZIP code, it's available in your region.

I'm trying to buy a new 4Runner with the tray right now, but it's not an option in the MN-WI-IL-IN region, so I have to talk to dealers in Iowa and Missouri or Michigan. Toyota is a real pain sometimes. Really ruining my shopping experience right now.