Hi kida,
Sorry, i work in both feet and inchs aswell as cm , mm but always Uk gallon, so my new one will be around 1200 UKG.
If its going to be poured then you/we could make the front/window aperture so the glass can sit in, say about 3x3 inch insett then silicone it in. Would be plenty strong enough, but dont hold me to that, LOL.
Just a bit of nifty shutter work involved in the making
I have been looking at pond seal so there would be no need to GRP it, the concrete needs to be slighlty damp also for some as it helps it dry. Well thats from what i can remeber from when i read about it, i think G4 works like that.