Square Raincover

I'm fairly new to myog, but hoping to make a rain cover for a square-ish travel backpack. (Patagonia MiniMLC) All of the diy rain covers I've seen have been fairly round, so I'm having trouble wrapping my head around the corner seams for a cubic shape. Does anyone have any suggestions/links/videos for something similar? Planning on liquid seam-sealing, so I figured flat seams would be best. Thanks