A home made holdfast seen as used on my anvil. Made from 1.5" sq. tubing for the frame. The springs are automobile valve springs. The cam I forged from a 3/4" thick piece of mild steel. It is secured by two 10mm rods in the base. To remove I just slide it off.

www.anvil.co.za/Tutorials/tut_tools_of_the_trade.htm

Here's a tip to add some weight and cut out some noise. I did it on my RT anvil and it works fine:

Weld a piece of angle iron on each end, so the I beam forms two dams, one on each side. The L iron should form a "hook" (my english just went out the door) so whatever you cast in there doesn't fall out. I.e., weld on the narrow end. A thousand words and one picture....

Cast these dams full of lead.
The lead will shrink on cooling, but a few hard raps with a hammer along the edges swells it out and you have a heavy, relatively quiet anvil.

Tiaan