This is probably a daft question, as I'm not terribly good at carpentry!

I want to make a small square window casement 400 x 400. My carpentry
skills are just about up to cutting the wood square and the right
length. I can even rout it to make a space for puttying in the glass.

Where I fall down is the skills to make joints at the four corners. Are
there any fitments I can just drill in to join the wood at the corners?
They would need to be fairly sturdy, as this is an opening casement.