I've not seen any off the shelf ones so I'm hoping to get someone to weld a bar between the rear turrets.
Make sure it's got spreader plates welded into the turrets and make sure the bar is large enough diameter and an appropriate gauge. If you drill through it to attach the harnesses (which you shouldn't need to) you should sleeve and weld any holes with tubing.
I really, really don't like this whole 'bodge a bar in for harnesses' thing instead of fitting a proper cage - it rather skips one of the huge benefits of having a cage - avoiding death in an accident. I've said it before and I'll say it again - do it once, do it all (cage + harness + buckets) and do it right - it's why it's taking me so long to scratch the cash together to do my 'safety package' on the Syncro.
Obviously the harness you're getting will have 3" shoulder straps, won't it?