Replacing Agio patio chair 'slings'

I have a set of Agio patio furniture which I like very much but the seat fabric has started to rip after a few years. I have removed a sling assembly and studied how it must have been manufactured. There are two curved siderail with a full length groove into which the sling fabric and splines slide. There are two 'stretcher' rails and a molded header. I figured the process was two slide the fabric/splines into the channel in each side rail, and then stretch the rails apart just enough to slot in the header and the two stretchers.

It seems I need a jig and/or a stretcher (eg screw jack type tool) to do this. Where can I get one, or how can I make one?

I can provide a photo.