http://www.polygonsolutions.com/

They do a basic holder, which they also sell via ebay, but it's still ~$400.
I have various links somewhere, along with a spreadsheet to calculate the estimated force required. I gave up the thoughts of making one when I realised the force required for what I was looking to acheive. Although the current lathe may actually manage it...I'll have to add that to my list of things to check when I get time!