Hot Tub Foundation

Hello everyone!
I am getting a hot tub and am building a foundation for it. I need opinions to see if this will work.

I bought (4) 8' treated 2X4 and will use a total of 12 galvanized nails to make a box out of them to sit on top of the ground. Then, I am going to pour pea gravel inside the box (and make it level.) I am going to put treated plywood on top of the pea gravel and the hot tub will sit on top of that. Any reason for this not to work?

Thanks all!