Advice on finishing an external window

[monaka19]

Hello forum. I live in an area very lacking in available tradesman. This may be a world-wide problem, I'm not sure. Anyway, I've been left (by my builder / carpenter) with a fixed window unfinished on the outside and I from necessity I may need to give finishing at a go. I have attached some photos. The exterior of the house is rendered. The window is currently sitting there with what looks like ply reveals, with a fairly big gap at the top where the lintel was cut in. The builder/carpenter said I just need a renderer, which may be true, but (a) I'm not sure if it's possible to render directly on top of ply and (b) I'm not sure of I need some kind of steel or aluminum trim to attach to the reveals or the top gap. Any advice would be appreciated. Thank you!