There's a clamping fixture that used to be sold out of the UK that's pretty nice. It's basically a set of vee-blocks that bolt face-to-face, and has mounting feet to bolt to the plate. I don't remember what it's called. I think it was named after it's maker. I'm sure someone on here knows what it's called. There was a guy over on another "hobby machinist" forum that recently posted pics in their shop made tools thread. I've been recently banned so I can't find it to provide a link. Otherwise vee-blocks that have holes through them can be used with an additional clamp. You can even use an angle block with U-bolts.