inside sealer for window box

Hello folks,

I'm making a window box for the front of the house and am looking for some advice about sealing the inside.

The box is constructed from 12mm marine ply and tas oak and is about 800 x 200 x 200. Beginner level joinery, ie. butt joints all round.

For the exterior I've applied F&W Woodshield which claims to give weather and moisture proofing while allowing some breathing. For the interior, I thought the marine ply would last longer if sealed against damp soil. I originally planned to use Bondall bituminous paint but I'm not sure if having an impermeable finish on just the inside would cause the long sides of the box to warp over time ?

Although this box is for flowers, I'm planning to make some more for vegies and strawberries, so I want to use a non-toxic product. The bituminous paint is apparently drinking water safe.

Any advice on the best interior finish would be much appreciated.

cheers
Michael