Double Shell

If one shell starts to taper (inward or out), then the laser that produces it is not perfectly vertical or the camera itself is not properly aligned vertically.  It may appear to be, but it's not.

I would suggest using a calibration object that is known to be perfectly vertical, and that has a slightly darker color.  The reason for the darker color is that it will reduce the brightness and reflection of the laser on the surface and create a visibly "thinner" line.

I use a checkerboard pattern attached to a CD jewel box.  When I open it up, the jewel box sits nice and vertical. It would also help if you find and mark the center of your turntable.

First, we need to line up the camera.  Place your calibration object in the middle of the turn table with it's vertical line aligned with the centerline of the turntable.  Next adjust the camera so it's vertical cross-hair lines up with the line on the calibration object. 

Align the horizontal cross-hair with the horizontal line on the object. DO NOT TILT the camera to adjust it.  The camera should always look straight, not up or down.  You degree of tilt should be 0°.  Use a square or protractor to make sure.

Once you have the camera aligned, tighten donw the screws just enough to prevent it from moving.  Over tightening can lead to misalignment.

Now you have to align the lasers.  first bring them as close to the center as possible so they intersect in the middle of the calibration tool, then calibrate each laser vertically one at the time by rotating them in their socket.  Again, tighten the screws just enough to hold the laser, but don't over tighten.

When you have everything lined up, do an auto-calibration, and look at the resulting PNG file.  Specifically, check the little green and yellow cross hairs at the top and bottom, they should ideally overlap, but if they're close, you should be good.

That's about all the information I can give you.  If you do this right, your final output should have only one shell.

Good luck.

Got it sorted out.  After going through the calibration again the x axis measurements changed.
The one laser misses a little more than the other but I can live with that.

There has been more than one calibration tool, so in the case of the original one with the 3 tips, then the horizontal crosshair lines up with the tips. 

There's another calibration piece that uses a checkerboard pattern, so you line it up with the horizontal line edges of the squares, and I've seen yet another 3D printed piece that seems to have some horizontal slots in it.

Whichever one you use, the principle is the same.

You need to move the camera further up on the mounting post so that the cross hairs are aligned when the camera is perfectly square.  The camera should not be tilted, it should be perfectly vertical.