I made my own for about $50.. I used a banner to line the inside so it'll be waterproof and bought some canvas and straps and buckles and you can get a seat belt from the junk yard. I've carried a 30 case inside and a whole bike strapped to the outside without problems

oh yeah and I used two layers of canvas to make sure it was strong enough because I just got cheap canvas from Jo-ann fabrics and you need to use a middle seat belt and you can find lots of gray ones