there's a few threads on this. many people have done them. I have done one as well. Removed the seats as well as the seatbelt hardware and seatbelts. I would suggest though using a 3/4" thick plywood rather than 1/2".
one thing that is not talked about in a lot of these DIY is the underside support and how much and where to place them. I probably did a bit of overkill and what I did is below before I sanded finished and sealed the wood. but that platform isn't going to flex or go anywhere no matter what I throw up on it. the 4 smaller holes go through rivnuts I put in to the support brace for the second row seats
3rd Row Deleted! w/ pics and info (**Updates on page 3)