Can anyone help me with this 3d model?

I made a model in SketchUp that I would like to 3D print. Of course I’m a complete novice with this software, but I tried to make a model anyways and learn along the way.

After importing the model into Cura it gives me a warning that my print isn’t watertight and that there might be problems printing the model. It shows me the following: (the red parts aren’t watertight apparently)

I don’t know what I did wrong and I’m too much of a novice to fix this. I tried searching to no avail…
It probably is something simple

The model looks like this:

This last screenshot is with the top layer removed. This thing is supposed to be a front bumper cold air intake.
There’s probably a million ways to make that rectancle to circular shape better, faster and easier but this is what I managed to figure out myself

Here is the file itself:
Test intake.skp (612.8 KB)

If anyone could help me with this, that’d be amazing
Thanks in advance!

I removed the surfaces and inner faces, and created them anew with Curviloft. This reports being solid.
Test intake.skp (207.8 KB)

Click in sequence on the Scenes tabs of this SU file if you want to use only native tools.

Circular to rounded rectangular adapter with thickness.skp (146.9 KB)

Thank you for this detailed instruction. I will try to make it like this. It’s very detailed for what I’m looking for

Ah thank you very much for this. Now I can do the final measurements before printing it. I’ll try and recreate what you did with Curviloft so that I can learn on how to do this in the future. Thank you so much again