I'm still trying to find a way of making sensor domes from acrylic sheet. After my last attempt destroyed an A4 sheet trying to make just one dome, I made an A6 clamp so I can make 4 domes per A4 sheet:
I still need to make a mold.
In the meantime, I discovered these:
On the left is a single seedling propagator that can be bought at gardening centres. They are not as hard as acrylic, but compared with the milestag dome on the RHS they are just about the right size. The flange they have at the bottom also makes for easy mounting.
They may be of use to someone....