First off, weather tightness is a major issue on motorcycle luggage. Things that would stay dry inside if you just left them out in the rain will get very wet inside if you drive at, say, 60mph in even a medium rain. If you're going for the 'expedition' look, why not ammo boxes? They seal fairly well. Check your military surplus stores / online.

Second, metal hose clamps are going to do a number on your paint (which you may or may not care about. There are hose clamps with fitted hard rubber pads that should at least help - or you can go to the local hardware store or auto parts store and get some rubber hose with approx the same id as the od of the frame member you're clamping and use that to make a pad. The fitted pads are less likely to slip out. Personally, I'd use metal strapping, bent to shape and drilled to take advantage of any bolts that were in the frame.

Finally, using those toolboxes, I'd mount 'em in as close to the body of the bike as I could. That might mean putting the latches on the outside (which is how most motorcycle luggage works anyway). The idea is to get them in behind the 'wind shadow' of your legs so you get less drag from them. Even so, expect to move your air drag center of pressure rearward, and be cautious the next few times you get up to speed as this might induce a wobble (which can be quite... exciting). Probably no worse than having a passenger, but just be aware, starting out. Also, that wind shadow will keep things marginally dryer in the rain.

I guess if I wanted cheap saddlebags, I'd go looking though the motorcycle jumbles (or online) for an old pair of bmw/samsonite bags, and weld up a set of mounts to fit, but my tools pro (even if not for your bike) you can make the mounts almost fit and then find a welding shop to do the rest for you. They're pretty inexpensive, and the guys who work there tend to be motorcyclists (all be in IMO primarily harley guys). Also, my experience is that if you bring 'em a six pack, they wont give you a better price, but will get to your job more quickly