I usually do a normal t-shirt neck finish - join the short ends, fold the strip in half lengthwise and sew the raw edges to the raw edge of the neck (so 3 raw edges together), then top stitch close to the neckband, usually with a twin needle. That gives a finish that's like most of the t-shirts I buy, and is far easier than I ever expected it to be. I do something similar but with a wider band to finish the sleeve edges of grown-on sleeves.
Both of those methods were done on this t-shirt, but they don't show up terribly well in the photo.
Strips are never on the bias, and usually 80% of the elngth of the neckline.