There are some decorative cement blocks that we used to use to make cheap bookcases out of. They are about 3-4" thick. I don't know if that's too thick for you. They would probably sell them at a big box hardware store.
Would some sort of metal grill work? I don't know if its heavy enough.

Real clay bricks could be attached with a couple of cable ties.

Thanks, guys. The bricks would add too much height. I got a bunch of crates and on their sides they are only semi-stable, and I want to keep them on the floor of the ex-pool.

Grills!!! Now there might be something there!! And very tie-able!! Catherinet, hmmm!!!! Now that is an idea!

Where to get heavy grills that no one wants, that will not pollute the water...

Sometimes I just walk through big hardware stores and look for things that might work for what I want. It usually doesn't matter what it was really meant for, as long as it suits my purpose. So you might try doing that. It can give you lots of ideas.
Or....you could just wait until you had the plants and pump ready, and set them on the crates. That's what keeps mine down.
My crates have holes on all the sides, so I can just turn them upside down to use them. I think that's more stable than on their sides. And your fish could still swim through them.

I use am using retaining wall blocks instead of milk crates. I noticed a bunch of broken ones and asked the guy about them and he gave them to me for free. For smaller pots I can use the ones that are broken in half standing on end. They may not be tall enough for you and it depends on how big a pot you are using. They come in black as well as lighter colors. Hope this helps

I picked up a couple of cartons of 6 and 10 inch square ceramic tiles that were on sale as discontinued. They cost about $.10 each. I use bricks, too but they are suspended on the inside of the crate so they work to keep the crate from becoming top heavy. They are wired in. I had to enlarge the holes in the crates because the fish got too large and kept getting stuck. Sandy

That's a good idea Sandy......putting the tiles on the inside of the crates.
One more thing to remember (from what I've heard)......be careful putting too much cement stuff in your pond. I think it can change the pH too much at first and it can hurt the fish. Am I correct here??

My understanding is that cured/aged concrete is stable and does not affect the water, although I am careful about making sure that if it has a finish on it I know what it is, acids and other chemicals may be used to wash concrete to expose surfaces. I generally soak anything I put into the pond for a month before I use it, quarentine same as fish or plants that I don't trust. Hope this helps

I got the idea for putting the crates on their sides (strapped together because our pond is so deep) from you guys -- was it Sandy? -- It was a good idea to let even bigger fish "use" the sided crates as a little shady "cabana" and the little fish can swim right through the holes as well.

Tiles might be good if you could somehow drill holes in them, just to make them attached to the crates. Hmm. I like the idea of just walking through a hardware store to see what would work.

Why I would not just use the pots is because I tried that! As soon as you lift the pot off, the whole double crate floats up to the surface.

And I didn't use brick because it's too heavy and I have to go too high.

Lots of good ideas here.