Sounds like a project for the new 770!
Looks pretty simple, and you've got one to copy. Just redimension if needed, or use the one you own. I don't follow your wish for "heavy duty". If you're doing roughly the same thing on the 770 as you are now, the forces on the cutter and work should be about the same. Within reason, as speeds and feeds may change some. But...if the vise is working now, try it. Don't fix what ain't broke.
I'd also suggest you look at a completely different approach. Hold your blank in a Wood's metal "vise", or use meltable/remeltable plastic (polycaprolactam if I recall correctly). Wood's metal and similar alloys are slick. Just build a cup out of steel or aluminum to hold it, diameter big enough to fit anything, external hold-downs as needed, and fill with the alloy. Heat to 100C, the alloy melts. Cool, it freezes AND EXPANDS (unlike almost all other alloys, which shrink on cooling). The expansion gets you a tight hold -works for milling & turning- as long as you don't take huge cuts. Melt to release. Infinitely reusable. I've got a 3" pot (and old spun stainless salt shaker base) that I use in the lathe for weird shapes. This is a common trick amongst the live-steam community, who need to bore/thread/mill odd shaped cast valves, for example.
PCL is similar, hold isn't as good, but it's handy for positioning if you have a way to hold tighter for machining.
Cerro carries these. Watch what you buy. Older ones contain cadmium and lead, and should be handled with awareness of those components (wash hands after handling, don't discard the alloy to a landfill, like that). That said, they're no less safe than, say, a fluorescent tube. Handle appropriately. You can pick the temperature you want, too. See what you can find surplus. These are not cheap alloys. Bismuth, indium, tin, etc make for an expensive purchase.