It's a good idea to stitch the two sheets of Silpoly together along the inner ridge line seam first and use this a datum edge for cutting the two sheets to size.
Then hem the short edges with 3x 20mm turns, opening the ridge seam at the ends as necessary to allow this.
The ridge seam is, essentially, a French seam with three web tags included for a ridge line to pass through. The slippery-ness of the Silpoly, now in multiple layers, becomes an issue for even stitching at this point. A walking foot didn't really work; maybe a Teflon one would.
Finally, hem the lower long edges to 20mm.
I used 15mm 'anorak snaps' for the cape front / ridge line edge and 1/2" plastic Kam snaps for the other three edges. If using two colours, as here, the Kam snaps need to be 'mixed and matched' (male and female) to use up an even number of each colour snaps, 3 at each end, 2 at the corners, 6 on the long edge.
Make up three 'tubes' of Silpoly for the pegging points and attach these to the hems with 3 buttonhole tacks (double bar tacks, or a buttonhole without the hole :-))