Sometimes we have to step away from the garden department to find something that will work in the garden.
The other day I was at Home Depot, near the concrete, masonry, tile supplies aisle and found the most useful item. It is a kneeler board to help the workers who install tile and such and have to spend a lot of time on their knees working on rough surfaces.
It's very sturdy, has two lovely handles, and a thick cushioned area. It's large enough for even my fat knees. It seems to be fairly water resistant but I probably would not recommend leaving it outside forever in the hot sun.
It's the Anvil brand kneeler board. Available from several retailers at a cost between $15-$24.
https://www.homedepot.com/p/An...