Floor soft and flexes

[steplift20]

I cut a piece of plywood and it fits nice,covers it good I know the right way is doing what phatdaddy said but it's not a floor that is always stood on so what I think I'm going to do is glue it down with a adhesive caulk , paint it and that should be good p.s. I did get fiberglass to fix a crack the is on the side of the floor so after I fix that I should be good, I hope