Amigurumi button/sleeve hole

This might be a weird one… I’ve crocheted a small amigurumi doll and am now attempting to make its clothes!
I want to knit just a basic T-shirt, ideally in 1 piece, which would fasten at the back, but I’m getting stuck as to how to make the arm holes.

I am using embroidery thread and 2mm needles, so I’ve worked out that my arm holes need to be about 9 rows high to fit the arm through.
I’ve tried traditional methods of making button holes but that leaves me with a tiny hole that’s too small.
I’m considering just making a t-shirt/sweater pattern on the smaller scale, and putting the arm holes on stitch holders while I work the shoulders etc. but would I be able to do this in 1 piece with the join centre back!?

I think I’m confusing myself… any suggestions would be greatly appreciated!

You could knit a one piece rectangle which is front and back with an opening for the head. Then pick up sts for the sleeves on each side.
Alternatively you could knit up the rectangle for the back, cast on at each side for sleeves and then continue over the shoulders and down the front, casting off the sleeves and finishing at the bottom front. Of course, you’d leave an opening for the head.