Pepper spray, improvised or not, would have to be damn strong to do any permanent damage. Even at the ridiculous concentrations of some hot sauces these days, I think the worst you would get is swollen eyes, nasal passages and throat. I suppose if somebody were already disposed to breathing difficulties (elderly or severely asthmatic) you could put them in the hospital. You'd need pure capcaicin to do any real damage...but pure cap is a powder and is a powerful alkaline on par with Lye which burns just as bad. I don't think you can buy it over the counter either.

I make liquid spray hot sauce for food (mentioned in another post)and you'd need a pretty good sprayer to propel it very far...it's pretty thick.

If I were going to do this (big if of course), I'd get my hands on pure chili oil extract (can be as hot as 6 million Scoville units last time I checked), cut it with something to make it thinner and then find a car parts store that mixes paint on site. Pay the guy extra to make a spray bomb in the smallest size possible with your oil.

As a Canadian I wouldn't bother though...my interpretation of the law here would make me extremely hesitant. Even though chili oil isn't a weapon it becomes one as soon as you spray it in somebodies face. Same if you smash somebody with a baseball bat.