do you think it can actually be worth to make such items? you can suppose that people that are willing to buy things to mount themselves, will probably be the kind that tries to make the more easy stuff themselves (unless you can make them verry cheap and professional looking). so the largest market would be in the more complex things, and the things that are visible( so people want things that look factory built). and there comes the problem; complexity and production time. ask any bodyworker to make you some wheelskirts for your car, painted and ready to mount. hell say "come back in a few days and bring a handfull of 100 dollar bills with you.
seems unlikely that such things can be handmade for a reasonable price. that is if you want to earn something on it.
the material cost of some of these projects alone is oftenly
and in these kind of branches the material cost usually only represents 10-20% of the total cost.