I'd look at something inexpensive, in terms of bags, that are locally available. As long as it's not flimsy, but all your gear can fit inside. I use the XL-BOB from County Comm as the main bag in my trunk. Fits everything I want to carry, and has room left over. (I think that's important. You don't want a bag that is stuffed to the gills because you went with a smaller size, and then literally have no room to add anything else if needed.)
Though that bag is around $40 and County Comm customer service has deteriorated very badly. Very badly....
LA Police Gear does offer a similar bag that is often discounted to $20. But I have no clue if the quality is as good as the County Comm one.
Looking back, I'd likely get a good North face back-pack as my Go Bag. (Perhaps the Recon model due to not only having shoulder straps but a waist strap as well.) If something happens and you need to grab your Go Bag out of your car, carrying around a (most likely) heavy bag across one shoulder is going to become a pain in the butt really fast. Still, a bag with one shoulder strap is an option. A good canvas Messenger Bag is a nice cheap option, depending on the gear you plan to carry. I've got the Rothco model myself. While the brand's products tend to be Hit or Miss (more miss than hit), their O.D. green canvas Messenger Bag is quite good and far from expensive. But I'd still check out bags and packs from local sporting goods stores, first.
Hope this helped.