A couple of years ago I made one for a round tank. It worked pretty well, but the materials I used for the prototype only lasted a couple of months. It's basically the way ShakeyGizzard described, but with the ability to flex to the round sides. I used flexible, plastic rulers (the old promo ones from Aquatic Eco-Systems) to make a sort of duck-lip scraper, and a plastic bag around them to guide the debris to the tubing. I used a rigid tube on the device and attached vinyl tubing to the rigid tube. We weren't dealing with coralline algae or anything though. You'd want a tougher scraper than mine for something like that.
I would be very interested in buying such a product. It was kind of a pain to make.