What can I use as a weight?

I want to try my hand at fermenting.
I live in a small town in southern Mexico and of course, there's delivery nearly everywhere on the planet now--but I'd like to use something local as my weight to hold the ferment below the brine rather than order online because 1) because that's the green thing to do and 2) this is my first foray into fermenting and I'm notorious at abandoning hobbies so it could also be my last.

If it depends on what I'm fermenting. There's a local red onion plus habanero pepper that is usually preserved in vinegar. My husband loves this so I thought I'd try a fermented version because we currently only rarely eat anything fermented and I'm looking to add some fermented foods into the family diet.

Can I use a rock? I assume it has to be something non-reactive, so not metal, and maybe not just any old kind of rock. I can find chunks of marble where I live that are about the size of the glass fermenting weights I saw on YouTube.  Would that work? I have a couple of very very small ceramic plates from Japan that will probably fit inside the mouth of a wide-mouth jar? But is that heavy enough? A shot glass? Is that wide enough to hold down the onion and pepper slices?

Feel free to jump in with any other bits of advice as well.

In an ideal world, you want something that is both non-reactive and not porous.

Unfortunately, a lot of stones like marble are in fact porous and could set you up for disaster.

A few suggestions as stand-in fermentation weights is listed here: https://culturesforhealth.com/blogs/learn/natural-fermentation-how-to-keep-fermented-vegetables-submerged#:~:text=Mini%20jelly%20jars%2C%20condiment%20dishes,to%20help%20keep%20vegetables%20submerged.

I'm kind of curious about the cabbage leaf technique. I might have to try it myself!

When I ferment hot sauce I cut some of the long peppers in half or quarters, long ways and use them to wedge all the other material down so I can't pop out of the brine. So I use nothing. It works for that but it's totally dependent on the size of your jars and materials. Just know that it probably evaporates faster this way so you have to watch your liquid levels.

I think you have a good idea with the shot glass, but maybe a larger glass like a juice glass or tumbler would be a closer fit, or a smaller glass jar. You could add some small stones for added weight inside of it if it floats (water would also work, but might evaporate away!).
If you have to use a stone, maybe coating it completely in wax would seal it up.

I have used a smaller half-pint “jelly” jar filled with water as the weight for a larger wide-mouth jar. A large cabbage/fig/grape leaf under the weight keeps small bit from floating up.

I picked up some beautiful river worn stones on the beach at our local river once for using as weights...the surface of the white ones dissolved in my kraut!
We ate it anyway as I had washed and boiled the rocks first but I haven't used them since.

I had previously and now again after the rocks use a jar as mk describes and it works pretty well.  Sometimes some bits get past it and I just fish them out.

I would love the ceramic weights or even glass ones that I've seen for sale.

I have a couple of big slabs of quartz that would be my first rock  choices.  Basically total non pourous and chemically resistant.  Second choice would be to go hunt up a couple of large quartzite gravel pieces as it is widely available here and also mostly non pourous and chemically resistant.  Third choice would be a glass punch bowl upside down rolled to let the air out.

I used narrow jars in the mouth of a wider jar many times. I've also used rocks, boiled to kill microbes, but they etch as the lactic acid builds up. It wasn't a problem for me, but I could see some people not liking it, and it might prevent you from reaching as low a pH as you might otherwise. The cabbage leaf works fine as long as the produce under it isn't cut too tiny, causing a million floaters that are impossible to wrangle.

Thank you you are brilliant!
I think I will try the shot glass--narrow jar--I have a few rather narrow jars olives came in that I think I can fit inside the mouth of the other jars. and I can put a stone inside if it seems like it needs a bit more weight.

Well dang, in four hours everybody said everything I would have said and the problem is already solved! What a delightful collection of lovely lunatics we have here!  

So, a postscript anecdote: When I moved into a 1930s farmhouse in '94, I was puzzled by a small collection of clean, rounded field stones in the basement well room, which was also a fine cold room. Anyway I kept them -- sensing a mystery to be solved. It took me years to realize these were kraut stones that had probably been used for five decades. Together they would cover the surface of a large ceramic crock, fitting together perfectly. When I moved, I left the kraut stones for the new owner to figure out.

I have a left field answer for this, which has been percolating in the back of my mind for years.

A Calcium aluminate cement such as RapidSet CementAll is food safe, so you could cast any shape you want.
They are  however somewhat porous.
I think beeswax would fix that issue, but as long as your ferments are healthy,  I don't think  it should matter.
A fermentation weight infused with lacto bacillus seems like a good thing

If you have them, the old ‘bail top’ mason jars use a glass lid that would make a decent weight.