Thought I'd share this in case anyone else wants a simple way to make a smoothly contoured, and more importantly recessed, shape that would make for a really nice ceiling for the interior of something, like a plane or even a tank.

(As viewed from the bottom, camera looking up. Glass panels are what you see on the right-side edge hanging down, that's why it might look weird.)

I'm sure this is something that's been done and is known about, but may be useful to new players.
1) What I used was the Fuselage Inlet (part id Fuselage-Inlet-1) and flipped it around, placing the hollow inlet facing DOWN (X: 90)

2) Next I rotated it 45deg on its Z axis (technically, SP decided at some point to change it to Y: 315 *shrug* but same difference)

3) Then I went about editing its shape in the Part Settings:
-Length: 0.2
-Rise: -0.45
-Run: -0.45
-Thickness: 60%

4) Now I went to the Inlet's side -- for mine, via clicking on the Backwards button -- and set it to:
-Width: 1.5
-Height: 1.5
-Corners: MANUAL
---CIRCLE
---SMOOTH
---HARD
---SMOOTH

5) Now for the magic. Open up Overload, and select Fuselage from the dropdown menu.
Go down to where it says inletThicknessRear and change that to 5.0 (it'll originally be 0.66).
Click on "OK" to save the changes.

That will give you the visual in the above image!

CAVEATS

There's a couple, most are minor, but one is quite a problem.
MINOR:
-Being rotated to 45deg, means all of the values are tied together unfortunately. However, it doesn't need to be rotated for this to work, so you could technically have a rectangle, as long as it's within the above's size parameters (keep reading for this to make more sense...).
-Also due to being rotated, the "Inlet Angle" setting is useless, since it works solely on the X axis (therefore, makes it crooked).
-Making any changes via the menu, after setting the rear thickness in Overload, will revert it back to default. Trivial, but just know that's why the visual "broke", not necessarily* because of what you modified; just change it back to 5.0 to fix it.

MAJOR:
-The issue becomes that if the size I've provided isn't what you need, then that breaks the visual. :(

In my quick experimenting, to try and find the limits.
The good news is it seems that you CAN change the Inlet's opening size to anything smaller than 1.5x1.5.
I checked 1.25x1.25, 1x1, 0.75x0.75, 0.5x0.5, 0.25x0.25, and even 1.25x1.5 (oblong) which those all preserved the smooth and fully colored visual! Also, the Inlet Thickness doesn't need to be 60% ('front' at 0.6 via XML), as it's the Rear Thickness that's key.

BUT changing the rear width/height to anything but the default 2x2 seems to cause problems.
There's some mathematical ratio at play here, but I've not determined it yet (and I suck at math lol) so I have no advice on how to scale it properly.
I did NOT test the "Length" at all, to see what that impacts. I did try making the Rear 3x3 and Front 2.5x2.5 and I... think... it preserved the visual, but hard to tell because while there was no "black/uncolored" area, a square shape in the center did show up heh

HOWEVER, therein if the solution, sort of... You can change the part's overall scale value (via Overload) to bigger or smaller than its default 1,1,1! Granted, it's not ideal, but it's quicker than trying to figure out whatever the ratio is... :}

Enjoy!