The question of canvassing a square stern comes up from time to time. I documented what I did. This is an upside down stretch. I think the photos are pretty self explanatory, but basically I stretch the canvas across the hull from gunwale to gunwale for 2-3 ribs just in front of the transom. Then put in cheater staples around the transom to hold the canvas temporarily. I put a clamp on the transom to attach it for stretching. Then clamp the bow end as you normally would do when stretching. Stretch as you would normally, watching to be sure that you are not tearing out at the transom. Once the stretch is done, tack or staple the canvas along the gunwales as you normally would. Then trim the canvas at the transom to leave about 1/2” too long. Take that extra length and fold it under to be even with the face of the transom. With a syringe lay in a bead of bedding compound or sealer of some kind (I use Sika 291) under the fold and staple it down. When that’s all done you can pull the cheaters, remove the tension and finish the bow. If you are working on a boat that has a trim piece around the transom, you can do it the same way, but attach your canvas directly to the transom under where the trim goes.