Not a bad idea if this were a larger planter. For something of this size though, and given that I have no silicone, I reaaaaally don't feel like buying any, hahaha. But that's making me think in-tank refugiums, which could be really cool in a freshwater tank.
I'll go with the plastic canvas, if I can find any. My alternative would be a plastic bathroom stuff-holder. I found one that could have worked, with some modification, but it was $7 and I figure the plastic canvas will be cheaper, and can be made to size. I'm needing more hides for this tank, so I figure I can use leftover canvas and make caves, and cover the outside of both the planter and the caves with moss. I've got other non-aquarium projects I might use the canvas for, but that's in the future.