Here is the write up for my commuting trunk to fit the rack I just made for the road bike.
The trunk is made like my panniers in that it comes off a harness real easily. The removable part makes it ideal for commuting. A shoulder strap makes is a breeze to cary around. Although the shoulder strap is permenently attached right now, it will get some clips in a little bit and be removeable. The materials are remnants and such a little bit on the lightweight side. 1.1 silnylon, 1.9? coated, and packcloth. The roll top closure is more for compressing the load then to keep it waterproof. Two zippered side pockets and a main compartment. There is velcro on the bottom and two straps and buckles to keep it firmly attached in the harness. I've yet to use it, but it should work.
Or the whole site
Total costs were about 15 for the rack and much less for the bag (it was mosly remnant) Dont know exactly how much it fits, but it fit my lunch box, scrubs and shoes with a lot of room to spare.
Not bad for four days of design and sewing.