If you mean the outside height of the inner matte board is smaller than the outside height of the outer matte board, then cut a spacer out of a piece of scrap matte to make up the difference and support the smaller inner matte at the bottom. Can't tell the ins and the outs without a program. Tape isn't very good at holding things up over long periods of time.
And use some sort of glass or plex glazing. The rigidity of glass is sometimes to be preferred when you have elaborate matte and art packages, as is adding a rigid support at the back of the package.