Hi! I'm wondering if anyone could give some advice on how they would approach the following part:

I'm designing a loudspeaker horn into which I want to have a section near the mouth that can can be interchanged with 3d printed port components for testing purposes, sliding in like a drawer. The idea is that we can experiment with different hole sizes and shapes in the insert, so the front surface of the insert has to match the shape of the horn exactly so that it slots in seamlessly.

The current horn model was built using a lofted surface and guide curves for the inner and outer surfaces, then thickened into a solid before adding the flanges, screw holes and other details.

Then I have made a plane in the direction that I want to insert this interchangeable port component, sketched a cutout shape (rectangle for now) and projected it onto the face of the horn. I have then used that curve to make a filled surface which I can 'Insert into new part'.

The catch with this method is that as soon as I try to make a corresponding cut-out for the insertable port component, I cut through the surface which then deletes the reference surface for the new part.

Any thoughts? Thanks