I use solid blue engineering bricks for trimming ballast. They are 3.2kg each, and I did weigh them to check. Seconds from a builders merchant are cheap, a Ton would equate to something like 317 bricks, which would cost, if I remember correctly, something like £60 as seconds, so its not a massive investment. They are also easy to tranport as you can move them in a few goes. I can't see the point in going for expensive storage radiator ones, they can't be much different density wise can they.