hanson,
Well, kind of a long story. I had a couple of bags of quickcrete sitting around that had partially hardened up and I didn't know what to do with them. So, I got the bright idea to use them in the ballast box. I knew two bags wouldn't be enough weight, so I bought two new bags.
However, I didn't want to have a permanent fill in the box, so I decided to line it with a couple of layers of sheet plastic. This also sealed up the box so that the liquid from the wet concrete would not seep out the seams.
So, the way I filled it was, put in the plastic liner, then, alternated layers of the old hardened up quickcrete (crunched up with a sledge hammer) with layers of wet mix (mixed in a wheel barrow) with plastic sheet in between.
This way, I figure that I can remove the concrete in the future if I want to by just smacking it with a hammer and chisel to break up the layers. However, in reality, I will probably never remove it.
Once I got the top layer of wet mix in, I smoothed it out with a trowel and made sure it was level. The four bags filled it just enough to leave room for the cast iron weights in the top.
The other thing I like about this solution is that it gives me a great place to store the cast iron weights which I don't use all that much otherwise.
On edit, I noticed that I wrote that I had 473 lbs of weight in the box with 4 bags of quickcrete which was a mistake. I think the number is more like 437 lbs with just the concrete and 809 with all of the weights.