Hi,
I would like to build a platform for a toolbox so that it follows me as I walk instead of carrying the toolbox by hand. It should be able to handle a 50-75 lbs payload. It will be on flat terrain all the time.
The best place to start searching would be in the Tracked Robots category. When viewing a product, make sure to scroll down the product page past the “Frequently bought together” recommendations to the detailed description, specifications and useful links which will have details required for you to choose your platform.
The least expensive platform we seem to have that approaches your requirements is RB-Zos-03 (max payload is 30 kg or ~66 lbs).
There are also many others available, such as the RB-Nex-83 (max payload is 40 kg or ~88 lbs).
Of course, these are simply tracked platform. You would still need to add many components for it to be a complete system that follows you around, such as various sensors to detect its environment and target (you).