Get a copy of the classic, Timoshenko or Timoshenko and Young, or Timoshenko and Woinosky-Krieger "Theory of Plates and Shells", McGraw Hill, ( I have the 2nd edition of the latter , 1959). You should be able to find a reasonable estimate from a wealth of formulae there.Or based on your very special boundaries, you may be able to to formulate and solve the equations analytically.
If you are doing any seious work in this area you must beg, borrow or steal a copy of this truly great book . It is out of print but Ebay, for one, is a source.