I bought a Casper "Casper" King mattress, foundation and mattress cover. 
First the foundation didn't come with the rest of the order so on the delivery
day they called to tell me and cancel the setup.
Then finally the foundation was sent directly to me....the delivery was scheduled. 

I put the mattress cover on the mattress and it is huge.  There is extra cover where
there is  no mattress. I had to wrap it around the bottom about a foot.  It had waves
of excess fabric which I feared would be cause for a bumpy sleep. I placed a cotton
mattress cover over the Casper mattress cover which disguise any folds of extra material under it.