Hi,
If I had to solve this problem I would try following approach. I would put the the boundaries exactly in the middle between vertical and horizontal stiffeners.
I assume you have a liquid on the inside, causing the lateral pressure to change in the vertical direction.
With the vertical boundaries you will symmetry on both sides of the boundary. With the horizontal boundaries that line will not have symmetry on both sides, but could be assumed in a first approach I think, when the distance between the horizontal stiffeners is not too large.
In order to get a better boundary condition formulas from Roark could be used, or a rough estimate with the differential equations for a bending plate. The latter could take considerable time, unless a publication with this can be found, which is likely, as this is a common problem.
Good luck.