A piece of 1" steel plate ( even mild steel) placed/welded on a stack of barbell weights will work just fine. The plate can be ordered from an online steel provider (get 4140 if ordering) or found in a scrap yard ( take whatever they have). The size can be whatever you want, but 6X6X1" is good. The weights can be found at the local thrift store, yard sales, your neighbors basement, or local trader paper.
Put all the weights on one end of the bar in descending order and tighten the clamps. Weld them together as good as you can. Take out the bar and set the stack, with the small weight on the bottom, on the 6X6x1"steel plate, and weld it to the stack. Turn rightside up and you have a dandy 100 to 200 pound anvil.