Have you considered hemp for your slipcover project? One of my favorite hemp fabrics you might like is the versatile, casual 11 oz. semi-bleached canvas. The natural color and slubby texture is ideal for creating a neutral backdrop for just about any home décor project: pillow and cushion covers, slipcovers and window shades.
Hemp canvas is durable but not stiff and scratchy like the stuff you probably remember from years ago. Today’s version is supple and softens nicely with wear and wash. You can throw a lot of wear at hemp and the fiber will remain strong.
And, here’s a tidbit that might surprise you: the color deepens slowly with age and sunlight exposure, resembling a subtle tea stain look. Although I have a few resources for hemp fabrics, I usually buy mine at Hemp Traders.