Wheel ballast – how to?

I know it is sacrilege, but I want to add 20+ lbs to each of my wheels. I primarily use the Outty for firewood logging and the wheel ballast would be an advantage. Garden tractors and farm tractors routinely do this. I do not want to fill the tires with liquid as is sometimes done.

Anyone have any links/ideas/experience with achieving this?

Thanks in advance.