Reddit mentions of Darice 10-Piece Square Plastic Canvas Shape, 4 by 4-Inch, Clear

Looks like plastic granny grate. Here's a link to some: https://www.amazon.com/Darice-10-Piece-Square-Plastic-Canvas/dp/B0018N29Z2/ref=sr_1_2?ie=UTF8&qid=1509738217&sr=8-2&keywords=plastic+craft+square

This is a common myth that hermit crabs NEED to submerge to drink/fill their shell/clean. They drink with their small claw, dipping it in the water and lifting it to their mouth. They don't fill their shell underwater -- they suck water in through their setae (tiny hairs) and this allows them to slowly add the water to their abdomen, maintaining their preferred PH. Filling underwater would mess up their PH, so they likely have a way to seal off their shell if they do decide to go under. It's possible a saltwater pool could help with mites, but they do fine with their smaller inner claws removing dirt and grooming themselves (you can see them if you look into their shell at the right angle). Also some species in the wild don't even have access to saltwater. So they don't need pools, but some crabs do like to submerge as long as the water quality is good. Spizam71 goes into a lot more detail about this on my post.

I set up some pools myself before I learned they don't need them. If you'd like to go ahead with it anyway I used two of these tupperwares, they're 4"x4"x2.25". Took me a while to find tupperware deep and big enough for them without taking up too much space (I have a 10 gal). They're a bit pricy though, and if you have more space I'd probly get something a bit bigger. For entry/exit you can get some craft mesh and set it up however you want, I just cut it to size and hot-glued it to the sides. Not the best method for easy cleaning though, I've seen others make a little tee-pee and drape it over the side. You can get some small rocks/gravel and put them in the bottom. A sea sponge is good in the saltwater pool for maintaining water quality and allowing them to crawl out, same goes for live java moss and/or duck weed in the freshwater (duckweed needs natural light, java moss doesn't). If the water quality isn't to their liking, they're a lot less likely to submerge (from this video). Completely change the water out once a week.

Overall they don't need pools, just access to freshwater and saltwater (even then they could probably get their saltwater from food). It is something for them to do though, so it could be beneficial for their happiness. Maybe that's why people recommend it.