It is quite interesting model. Lower part is circular – we can use revolve to make it. However upper part is rectangle, so we need to find the way to join both – completely different – parts. Then another challenge – how to fit side stiffeners to such complicated geometry. It is easier than You think. And the most important – we need to fit it to existing filter holder.
Firstly we need to draw reference geometry. It is really important to measure properly.