You could use a simple stuff sack or drybag. They come in all kinds of shapes and sizes. The ones that come with some of the inflatable mattresses, or the stuff sacks that come with small, single-person tents have a good shape. You can just strap it to the rails under your saddle.
It might take a little tweaking and creativity, but you can make it work and make it look good.
If everything is rolled neatly in something that will hold the shape, and then the whole rolled bundle is inserted into the stuffsack (like packing a small tent), it will look good and hold its shape.
Then you could use a couple of the side-release buckles that come with some accessory straps (backpackers use these, and they are sold by companies like REI). Or you could use some other lashing system. Parachute cord (or something similar from REI, etc.) with trucker's hitches (aka power cinches) would work just fine, and -- like the side-release buckles -- would be make the whole thing easy to attach and detach.