So I need a whole bunch of snap cap 12 gauge shells to practice reloading with. I'm planning on just taking my spent shells, de-priming them and filling them up with something, painting them?.

But what do I fill them up with? I need to make about 20, I'm not sure of the internal volume of a 2 3/4" shell but I'm guessing at least 1oz per shell?

Was thinking caulking filler of some sort, but really have no idea. I have no need to match the weight of a real round, just something that will be durable and preferably hard but matches the internal volume once set (if it expands it will end up useless).

Any other tips for doing this? I'm a tad worried about the expanded size of the shell considering they are spent shells.

Why on earth would anyone need 20 dummy 12 gauge?

I explored similar options but with limited people attempting this and not wanted to waste extra supplies or time, I ended up just buying. The ones from brownells work good and have held up to my abuse. If you find an solution I would be interested though. I just couldn’t commit the effort.
30$ for the box

3-gun is the most common reason. Need to practice rapid shotgun reloading. https://m.youtube.com/watch?v=lprcnmeql2Y

I took 20 spent shells, and loaded a new wad and shot, and then crimped them.

Mark them good so there is no way to mix them with live shells and store them separate. This has worked really good for me.

^^ this.
Then I glue a piece of "O-Ring " cord into the primer pocket. Or even silicone/RTV.
Then you can safely dryfire the gun also.