Well I've hit on the brite idea of.... layering. Not gonna use round glass but will put square bit in between 2 layers of 20-25mm wood squares (not decided what thickness yet), halved and half circle cut with jigsaw (laff away but I bet it'll suffice). Narrow wood strips, same thickness as glass, around the glass in the gaps tween. Screw the wood together. Hey presto, round window. Will add more wood to thicken. Mastic around the glass/wood. Fit into 'wall' I'm building.This is only for a shed/hut, it aint' gotta be flash. Thing with screwing is (no, not THAT kinda screwing) is if glass breaks/cracks in future, I just unscrew the inside wood layer and put another bit of glass in. Give me a couple of years hehheh, and I'll have pix on here for you to either laff or praise. Ta for tips guys. And no, I'm not on acid at the moment : )