Summary

I needed a straight edge around the vent hole.
I also wanted the hole to be the same size as the inside hole of the vent brick and small enough for the screw holes to be in firm enough plaster away from the edge.
I threw this together so I could put this insert into the wall with plaster adhesive around the edge and set it in place, then plaster up to the edges.
It makes it look better, strengthens the edges and gives you a template to drill the holes for the screws.
For the sliding vent with hole centres at roughly 179 x 146