This trap is meant to be attached to a plastic bottle and the idea is to make it easy for the fish/shrimp to go through the cone and into the bottle but can't find it's way out.

The topmost bridge - called bait holder - is meant to hold the bait (you attach it with wire or any other means.

The bait holder is hollow from the inside. This is both for saving plastic and for filling it later with sand and seal the top hole with hot glue to make the trap heavier to sink underwater.
A tiny sand filter design is included with the project for your convenience.

After cutting a bottle's top part, you can put the trap inside it. Any plastic bottle would do even if it's opening is 30% larger than the trap. Plastic bottles tend to shrink to more than 50% of their original size with heat so the idea is to put the trap inside the bottle then use a candle or lighter to carefully heat up the edge of the bottle to shrink it and make a tight grip around the trap.

Here is a short youtube video showing me testing the final product
https://www.youtube.com/watch?v=ycHzinhyj5g

So does it work?

I've tested it with fish and it didn't seem to work. Fish were crazy about the bait but wouldn't go inside the cone under any circumstances.

I've punctured so many holes in the bottle to make it look as safe as possible and let water flow through it but still fishes would avoid going inside the cone.

I didn't get a chance to test it on shrimp as of yet so I am keeping it as "work in progress" for now in case someone decided to test it for me.

Notes:

• The trap needs to be printed with 100% infill because any air gaps would cause it to float. I took that into consideration while designing the trap.
• The base has a short end and long end, the long end is meant to be pushed in the sand or squeezed between rocks or attached to heavy weight to offer more stability
• I've used a hot glue gun to slightly melt/smooth the inside and outside of the ring near the base. It both provided more strength and the glue residue made the trap grip better with the bottle.
• For filling the inside of the bait holder, I've used a mix of sand and gypsum (50%) then added water and left it to dry then sealed with glue gun. it worked out great and made my trap sink like a rock and at the same time didn't absorb any water