The jar I used looks like glass but it's actually plexiglass, which means we can drill a hole for our air tube. 

Drill a 1/2" hole for the cable gland. This is easiest if you start with a smaller pilot hole -- the plastic is less likely to crack if you come at it gently.

I originally drilled two holes as I had a different prototype idea in mind, but ended up sealing up one of them with RTV sealant and only using the one hole.

Make sure the gasket is in place on your cable gland. Screw the gland in place on the jar and tighten the two halves together with a wrench or some pliers, but don't tighten the rounded end-nut yet. We'll tighten that once the air hose is in place.

Put your head image in the jar on the side opposite the cable gland, and secure it down to the bottom of the jar with some hot glue so it doesn't float up. Slip the air hose through the hole in the cable gland and leave enough room to reach the hole.

Tighten the nut on the cable gland. This will allow air to flow through the tube into the jar but not allow water to leak out around the tube.

Note: this image shows two cable glands but you only need one - the second hole ended up getting sealed up on my project.

If your lid has an airtight seal, drill a hole in the lid above the waterline. This will keep the additional air we're pumping in with our pump from pressurizing the jar and blowing the lid off.

Cut a piece of EVA foam or similar material to create a base for the jar. The Circuit Playground Express and MOSFET get hot glued to the bottom of the jar with the Circuit Playground Express (CPX) right in the center. It needs to be directly in contact with the jar so that when you tap on the side of the jar the CPX is jiggled too.

Hot glue works great for this. I covered the NeoPixels on the CPX in hot glue then pressed it against the bottom of the jar. The hot glue still lets plenty of light through.

The EVA foam is thick enough that the CPX is not touching the table and will therefore be safer from leaks, and the jar sits comfortably on the foam instead of the controller.

Glue the end of the tube to the back of the image using RTV silicone sealant. I initially used hot glue, since it builds up nicely around the tube, but the smooth surface of the laminate kept the hot glue from sticking very well and the tube kept pulling off.

If you get glue over the front of the tube, re-poke the air holes with your pin.