I looked into building some for loading our big lathes at work.
These are the portable ones with a concrete filled box base that can be moved with the fork lift. They are rated for 300 lbs best I remember, but that is a very conservative number, I'm sure they will go probably double that if ever needed.
But, after penciling it out, we were way ahead just buying them instead of building.
That also puts more liability on the mfg should something ever happen.
We got one at Global Industrial, then I found a better deal somewhere for the second, don't recall what they cost, but wasn't too bad.
Then I had to find a trolley and small hoist for each.
They work well, sure beat having to clear a path for the forklift and rely on someone else operating it.