You didn't say if you were looking for a knit or crochet pattern, but that sounds like the box stitch. Here is a link to a corner to corner afghan done with it, but you can just go until it is as wide/long as you want and then quit.

Here is a link that might be useful: corner to corner

Check out the "easy knit" and "triangular" shawl patterns in the link below.

Here is a link that might be useful: shawl patterns

I wonder if you are still in need, and if you'd like an easy triangle crochet pattern. The only stitch in it is a single crochet. Talk about easy! It was my first crochet project. Using 2 skeins worsted yarn and J-hook, make a chain the desired length for shoulder edge. I used a chain 70" long. Start in 2nd chain from hook. *decrease in the 1st 2 stitches at the beginning of the row. sc across. decrease in the last 2 sts. ch1. turn. repeat from * until reaching the point of the triangle. Trim as desired. I fringed my first one, and did a ruffle on my 2nd. It's thick and warm with no holes.