Yep, the DIY are easiest and cheapest. A few tips: Lightly sand the flat backs of the poster framing and make sure you clean the surface with alcohol first. Search around and you can find black craft mesh otherwise you can spray paint clear mesh. Another tip for that...lightly sand the craft mesh so the paint doesn't flake off.
If you are handy and have bigger tanks acrylic is the only way to go. Buy a sheet from the local home store, score it to size, snap it, drill holes(little tip here-drill through masking tape/remove the tape afterwards). FAR more forgiving that trying to get glass to cooperate IMO. You can make it snug enough to not need silicone or you can silicone both edges once it's in.