A few years ago, I read up on mason bees and what makes them reproduce in your orchard. Turns out that making mud available may be just as important as having "housing" for them. When they don't have mud, they will fly away until they can find some to pack their eggs in and then make their nest in that area. That is why they are called mason bees. I started putting out little plastic tubs of mud for them. Then I found that the tubs would fill up completely with water on a rainy day and I would be drowning mason bees, when they couldn't get out. I ended up drilling a small hole, about 1 or 2 inches up from the bottom, to keep some water in but keep the top of the mud above it. I also try to put a stick in each one, so they can climb out if needed. Then I realized that over time, the mud would be covered with leaves, moss, and pine needles. The mason bees had lost access to the mud! It was too hard to clean the tubs, so I figured I needed a quicker way. I decided to use the tubs that are wide enough to simply brush my hand across the top, removing the debris and ridding the moss. Then I found out that the tubs were drying out at times. Now in the spring, when it's dry, I will go out with a long necked watering can and make sure there's enough moisture in there. Just recently, I have become a lot more concerned about the amount of plastic in our bodies. It is flowing in our bloodstreams and getting stuck in our livers and kidneys (it's not just the turtles). I don't want to damage my health, nor the health of the wildlife. I have been raiding the family recycling bin for cans that are wider than normal, so I can fit my hand in them and sweep off the debris and moss. Then I drill them. It's a gradual process, but it makes for happier bees, more fruit and a more ecological orchard.
What a nice idea to provide mud for the bees. I’m just thinking out loud here, but maybe you could just add some mud on top of an old piece of carpet? Too much rain will drain so no drowning bees, while the carpet will still retain some moisture for a bit. Put the mudpiles in shady places to prevent them from drying out faster.
You could maybe also try the terracotta plates you usually put underneath terracotta pots, if you want to avoid plastic. They are shallow and there shape allows the bees to just climb out if they fall in the water. They might even be able to sit on the edge of the plates to drink the water, without falling in. So they could serve a double purpose.
If you’re still worried about the plates being too deep, you could cut up a few cords in half lengthwise and place them on top of the mud. If the bowls fill with rainwater, the corks will float on top to create little rafts for the bees. You can see it in this video. They didn’t cut the corks in half here, but I would recommend doing so so the corks won’t turn around like a log in water when the bees are trying to climb onto them.
Maybe I didn't explain the holes in the sides of the cans well. The holes are 1 inch/2.5 cm from the bottom of the can, so some water will stay in the can so it stays moist, like mud should be, but never becomes too hard and dried out for the bees to pick it up. It's still clay mud.
The point of the hole is so that only a maximum of 1 inch of water will be in the bottom of the cup. When the water in the cup rises to the level of the hole, it will drip out of the hole. It will drain out so that the mud "mountain" always rises above the water. The mason bee can pick up the mud by landing on the mountain and fly away. The water will never cover the top of the mud. They don't drown. The stick is extra insurance to make sure they can climb out.
I like your idea of terra cotta. It's a good material. No plastic there. some people have put rocks or sticks in there. The corks in your post are a good idea too.