Tried it all (almost)
Drone - I've played with several types of ballast (but now run without it as I've 'adjusted' to just the weight of the dog).
Chains in long bags work, and are less destructive than solid lumps of 'something' if things go wrong. I tried sand and some bags of steel washers. Takes up too much room. And I don't like 'loose' stuff as ballast. The chain worked as I could run it length-ways, and would be good in an inner-tube. It still bounces around though, but is not 'destructive' like a solid (unbolted) bar would be.
But - The reason for the post is that the placement is as critical as the weight itself. I'm sure you have considered all this, but it took some experimenting for the thought to dawn on me! You need to remember that 50 lbs placed between the chair and the bike is not as effective as 50 lbs placed on the outside of the chair near the wheel. That's just basic engineering. The further out, the better. The closer you can get it to the wheel, the better, as it has less chance to upset the balance.
I'd be looking at running a tube of 'something heavy' on the underside of the chassis. You seem to have the space on your rig. Even a couple of heavy steel 4WD 'helper' springs (the main leaves which are fairly flat) would be a lot of weight but have a good shape and mounting holes built in. Or a solid bar of steel in an appropriate length.
Anyway - just a few more thoughts. Best of all would be a 'plate' securely bolted to the underside of the tub.
And so ends my 'Adventure' era (SOLD)
IBA # 39193