I'm going to do it in the round - increasing at the corners.
Here's an example from Caron
With RS facing, pick up and knit sts evenly around entire outside edge, approximately 1 st per st on CO and BO edges,
and 3 sts for every 4 rows along sides; place a marker (pm) in each corner st and move it up each round; pm for beginning of round.
* Knit to 1 st before corner st; k1-f/b, knit corner st, k1-f/b (2 sts increased); repeat from * around.
* Purl to 1 st before corner st; p1-f/b, purl corner st, p1-f/b (2 sts increased); repeat from * around.
Rnds 3 and 4
: Repeat Rnds 1 and 2.
Bind off all sts loosely in knit.