...a variety of things. Depends partly on where the weight goes, how the weight is fastened in, and how much you are willing to spend.

Some examples:

Small balls have more density per cubic volume than larger balls because there is less voids between them. For this reason I use steel BB's. Just pour them in then pour some sealer. I use epoxy.

I made a small box (so the weight is removable) and filled it with hardened concrete nails. I had no need for them. Added some silicone sealant to prevent rattling then sealed the box.

Guidelines:

The best weight is the most dense (eco friendly) material at the lowest point in the hull. The object is to get the C.G. as low as possible. For fixed in ballast, the optimal would be to pour the material directly to the inner side of the bottom and harden or fill any loose ballast with something.

Use a little less fixed ballast than you need. This way you can "trim out" (level) the boat with removable adjustable (velcro?) ballast should you make any changes to the boat.

Hope this helps.

Lew
Florida ⛱️, USA 🇺🇸

I've used shot - #12 bird shot as I recall. Put suitable weight of shot in a plastic baggie, pour in just enough West System epoxy to cover all the pellets, and set in its final resting place for ballast inside the hull. It will harden in place and the baggie can be removed for storage when not sailing.
-Marty-

Gun shops usually have lead shot.
Roy

Hi

Like the others I find sheet lead is a good option as I can usually get it to conform the shape of the hull. The other option I use is lead shot which I bought many years ago. I understand it may now be difficult to get. I place a plastic bag or usually some kitchen wrap doubled up in the boat and fill in with the right amount of shot. It is really good for getting into difficult spots that lead sheet cannot and it often means you can get the ballast lower in the boat which can be an advantage. Once the lead shot is in place I seal it with a layer of epoxy or silastic.

However, I try to use the minimium amount of ballast instead using as many batteries as I can to provide the vast majority of the ballast.

I use roofing lead and rolls of old lead from gutters.
Roy

I use roofing lead, I have a lot of offcuts and what is left of a roll from alterations were done to my house a few years back. The roofers wanted to take it away so I asked them to weigh it and credit me for it as I was been charged for the roll. They moaned a bit but left the off cuts and remainder of the roll. This will eep me going for quite a while.
Another source is discarded wheel balancing weights, you may be able to get some from your local tyre fitment centre.

As with casting bullets, in US, go to small scrap dealer, and ask for roof relief lead. It's the lead tube used to allow a sewer relief pipe to penetrate the roof. Roofers collect used lead and sell to scrap dealer. Lead is pure, easy to melt (another story) and retails for about $1.00/lb in US. Melting lead is pretty dangerous, read up and be cautious. 😀