I have one of the Hare & Forbes angle vice clamps, AC-100, which I bought for $90 at one of their sales. Just checked it now for square and it's spot on. Maybe I was just lucky? It's a heavy casting and I don't always use it for corners but it certainly won't flex. If I'm building SHS frames, I'll generally clamp the pieces to my welding table, use magnetic 90s to tack corners and do a final check everywhere for squareness before completing the welds. I certainly discovered that cutting 45 degree mitres on SHS in the bandsaw didn't result in 90 degree corners when assembled. As my Tafe welding teacher used to remind us, MIGs are great for filling.:U