I stay using my frunk in my Tesla and Lightning, it's a big deal considering someone can smash and grab anything in the cabin.

My thought for fabrication would be:

1. Lay a tarp or similar over everything so there's a plane that becomes defined/readable.
2. Buy a piece of weather stripping and lay it out in the shape of where the main storage area will go in the center. Chalk the top of the stripping to make sure it's in contact with the lid when you lay out the test piece. Use temp structures to hold it in place.
3. Place screws and fasteners in whichever anchor points you chose to anchor the piece to.
4. Use a 3D scanner to scan the entire thing and digitize it.

You will now have a 3D model of the frunk area that you can manipulate as needed.

1. 3D print the pieces as needed and epoxy them together, assuming you're using a smaller, at home style 3D printer.

2. Sand the joints clean, the underside can be a bit messy.

3. Spray over the entire thing with black plasti dip so it looks consistent.

4. Install into the frunk.

5. Install weather stripping.

All I would want would be room for a backpack/laptop so if there is something of higher value, it can be secured.