Thanks Dave

The inside base is even with the top of the bottom rail. Loose fit (no screws, nails or glue) boards (front to back) along the length then covered with landscape fabric.
I did not install a false bottom half way up due to the design. I didn't want to attach anything to the inside of the T&G since it is a large floating panel.
I left plenty or room for expansion on all sides and the bottom, so far so good.
The larger one has been out in the weather for 7 months and the little one 3 months, no signs of wear so far, fingers crossed.