Note: Either with selvedge or no selvedge sts, my armhole is still bigger than the other one.
One option is to use needle and thread to sew up the difference.
Second option is don’t join the 5 provisional co yet. Just join the triangle and knit in the round. After the section is finished and BO, measure the edge of the triangle and pick up the center sts to join the 5 sts.

It is a one piece, seamless, top-down shrug/poncho. First section is a stockinette triangle knitted flat which you can try it on before working the next section. Second section is knitted in the round. There are instructions for various sizes; 30” – 32”, 34” – 36”, 38” – 40” and 42” – 44”. Any yarn and colour will be great for it.

My stockinnette gauge: 2” = 9 sts x 12.5 rows

Measurements lying flat:
30” – 32”: 38” x 18” (approximation)
34” – 36”: 42” x 20”, armhole 12”
38” – 40”: 46” x 22” (approximation)
42” – 44”: 50” x 24” (approximation)

Many thanks to testers:
Chrysopal
JTKnitterSan
knitintensity
cpredmor
wendylinjohnson
thestitchinwench

Note: You can sell your finished item.