1.)
You could try bending or rolling the last 3/8" of each piece over. One turning inside and one turning outside on both pieces. To join the pieces slide the outside turn of one piece into the inside turn of the other piece. Do the same with the remaining two ends.
You may have to put them both together at the same time which could be a bit fiddly. Also because the ends are simply bent over and not "shaped channels" it may only stay together if it is forced to be held to a tighter diameter than it would normally be if not joined into a circle.
2.)
JB Weld could be used in a lap joint on one of the areas where two of the ends join together. Use the rolled / bent / folded technique on the remaining two ends.
3.)
If you were to hem the top and bottom edges by folding them over on both pieces you could use the old slide tab A into slot B technique. Trim off a little of the "hem" to form a tab on one of the ends of each piece of Fosters can. Leave the "hem" on the corresponding piece of Fosters can to receive the tab. Don't fold the hem to tightly on the extreme end of the material or the tab will not join easily.
On idea number 3 I'm not sure how it will affect rolling up the windscreen for storage. I suspect it would try to kink in multiple places if rolled too tightly.
Also you could mix and match techniques from each of these three suggestions using your imagination. ;-)
MYOG is as much about creativity as it is technique.
Party On,
Newton