if you wanna let the light in, you could use chicken wire.... you know, the type with the tiny squares ^-^;
I used it for my 15 G with my frogs and it works excellently. you just cut a rectangle bigger then the tank on all sides, find where the sides are and mark them somehow on the wire, then bend the wire there to make edeges in the shape of the tank ( I did this by stepping on it, because it's super effective and if you use good shoes won't cut you X3; )
you just do this to the front and back sides, and leave the left and right alone except for folding in the sharp edges so no-one gets cut.
that way the front and and back sides hug/latch onto the tank, and you can slide it left and right to open it up ^-^ or visa versa you can only do it to the left and right sides, so you can slide it frontwards and backwards.
if you don't wanna do that much you could also just cut a rectangle a little bigger than the top on all sides, fold the pointed bits under/over, and sit it on the top and put something on one corner to hold it down. ^-^
if you wanna try that I can get a picture for you if it'd be more useful. ^-^; the canvas should work just as well with air holes poked in it though I think ^3^
for the plants could you maybe plant them around the deco? or baffle the filter perhaps ^-^; doesn't it catch your fish?