I have a 1940s gymslip pattern I got off ebay. It's for age 5-6 but is really easy to adapt sizes. I'm not sure how it'd alter for bigger chest size though.
The pattern I've got isn't quite the same as it's just slip over the top, no buttons on the shoulder, although I have done one with buttons for a change.
The main thing is to make sure that the neck edge isn't too wide or it slips off the shoulders.
I've made both my daughters grey gymslips for school from it, as well as prettier colours.

Ah, that's a helpful bit of info.

Essentially it is a long rectangle that has been sewn together to make a continuous body piece, with 3 big box pleats in the front and 3 in the back. This is attached to a yoke with a square neck, and a little bit of shaping is needed under the arms. You can insert a pocket into the side seam, or make the shoulders more adjustable by having a button hold the front & back together - the ones I wore to school were just straight over the shoulder.
The belts we wore were the same material as the gymslip, and had a button fastening. The belt had one belt-loop at each side.