If there's a "standard" diameter it's 5/16" / 8mm. I got myself a couple of five or six foot carbon fiber tubes, 8mm diameter, from GoodWinds kite shop. My "jig" is just chunks of 1/2" square aluminum extrusion that I've glued up. Put the ribs on the rods, set the rods on the blocks, all on a dead-flat surface, and you've achieved what a jig should achieve.
If you fear (or observe!) the rods not sitting firm on the blocks, a bit of masking tape or weights will hold things down.
I've used 1/8" music wire rods -- the 8mm CF tubes are much more rigid, and make it easier to build a straight wing.
Or just block up the trailing edge and be happy:
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