[Salem13]

Personally I just use straight up cardboard or popsicle sticks. I like cardboard the best specifically because I already have so much of it, it's easy to work with, and my hamster loves to chew it. Once it gets too chewed up or if he pees on it I can always just make another one. If you want to make them more 'pretty' you can use wrapping paper.

You can use a toilet paper tube for a water bottle holder too, just poke a hole about a third of the way up for the nozzle to go through and maybe a piece of cardboard on the bottom to stabilize it more. Or you could use a bowl.

The mesh I use for my dwarf is 1/2 inch (just over 12mm) and is about as big as I would personally go. He can fit his nose through.

As for a cage door you may have the best luck looking for secondhand and/or broken cages.