Foundation weight kit

Yes, they're are a few that make them. River city makes a bunch of different weights
 
I bought some Brass Hex Bar off https://www.onlinemetals.com/, for $40, cut it to size on a cutoff/chop saw, and dropped them in with silicone and called it good.

Do they weigh what I wanted? Yes. Are mine all perfectly cut, neat, and shiny? No. Can you see them at all? No.

I have a straight taper barrel, so it already weighs a fuckton, so I didn't feel like I needed to get super fancy with tungsten or some other expensive, heavy metal to add a few more OZs.