I been thinking............on ebay you see alot of 4140 in good size diameters, this stuff can be heat treated with a torch and water as I do my hammers. you could take a piece of this and make your own such as a 3" round or larger and sink it in concrete,
my opinion is that a hammer head is no larger than 1.5" like on my japanese hammers so do you really need a large surface for small knives,,,,,,,,,,,,i would think not. I have never done this but I am sure it will work, I also know that it is better this way than a harbor freight hammer.
I do know a guy in fayetteville, nc that has a bunch of anvils from 55 lbs to 300, but he wants top dollar, I saw the 300 lbers and are really nice, if I didnt have a 250 lb I would get one from him.
just my two cents worth.