Backflush adapter for original LifeStraw

I imported the .stl of the original creator's bottle cap into Blender. I Boolean'd a cylinder to the top of the cap.

Then I made another cylinder with a slightly smaller diameter. (To give a reasonable wall thickness for a sleeve.) I made a Duplicate of that cylinder for use later.

I used this smaller cylinder to Boolean out the top cylinder, above, making a sleeve for the TPU insert to come. This assembly is the main .stl, a remix of the original.

I scaled down the diameter of the Duplicate cylinder a bit so that it would fit in the sleeve later.

Next, I pushed the mouth of my LifeStraw into an ink pad. Then I stamped a piece of paper with the inked mouth, leaving an ink outline of the lip of the spout.

Then I used my camera with a slight telephoto (to minimize parallax) to take a photo of the outline. (Using a flat bed scanner would be best.)

I imported that photo into FreeCAD to make a reference image. I calibrated the image.

I traced out the left half of the image using B-splines. Then I hacked my way through making a mirror image of the left half of the points to make a symmetrical mouth.

I padded this sketch to make a model for a “hole” that I could use for a Boolean operation in Blender. (Later I found that I could taper the pad in FreeCAD to match the taper of the mouth of the LS. But it requires a hack. So I did it in Blender.) I exported the pad to .stl.

I imported the stl model of the “hole" into Blender and tapered the “hole” using Bender’s Simple Deform.

Then I Boolean'd out this hole from the Duplicate cylinder that I had made above.

This made my TPU insert stl.

I cleaned up the .stl's using Orca's Fix it. (If you don't use Orca, then I recommend cleaning up the .stl's using a free online .stl Repair Tool.)

I printed the bottle cap remix using PETG. I glued the TPU insert into the PETG sleeve.

I also made a washer out of TPU so that the bottle cap proper sealed better with the bottle.