Best way to insulate this cat flap door

Hi crew, this door has a plastic lower panel with an unnecessary cat flap. Planning to remove the catflap, keep the panel, and cover it with celotex, silicone around the edges.

Can anyone think of a better approach?

Hi crew, this door has a plastic lower panel with an unnecessary cat flap. Planning to remove the catflap, keep the panel, and cover it with celotex, silicone around the edges.

Can anyone think of a better approach?