Understanding lip and groove mechanism

Uchenna

Consider both sides of the assembly. With a lip and groove joint on both sides, they together align the parts. This works if the walls are relatively stiff. If the walls are flexible enough, then some details such as you show in your sketch may be required. The same function may be done with internal ribs.

I wouldn't say "hold them together" since, as I am sure you know, the two parts will fall apart unless fastened together in some manner. I would say "keep them aligned".

Dwight

Lip/groove is usually L shaped, not U shaped. There is usually a little bit of an angle just due to draft in the mold. The snap generally doesn't come from the lip/groove, but by passing undercuts in the parts, and there are several ways of doing that. Real seals generally use o-rings or gaskets - a compressible waterproof material of some sort, rather than relying on rigid material to rigid material seals.

Acrylic can be brittle, and is usually used for its transparency. There are a lot of materials available. You need to have a full list of your requirements, volume, color, properties and your cost targets to select a material.

Firstly, do not use acrylic, it is brittle.  If you wanted to use Lexan it could work or maybe some sort of ABS.

There possibly too many ways to snap-fit the two halves together, many suggested above. Watch your tolerances and shrinkage of plastics used.  Undercuts may work but could feel sloppy.  A better way might be a molded tab with a bump fitting into a matching groove or small pocket.

What you show is for aligning and glueing the two together.

Of course you can make it press fit.

Try snap hook and snap hook groove feature.