I used hex flange head self drilling screws too. They are not flush, but I never stripped one. I have a magnetic drive that holds them on to the hand drill. I didn't make a bar like you have , but just twist wired the coroplast sheet to the suspension beam. This covered the back 8 feet of the underbody. If screws get stuck on removal, they just break off. Remember that the suspension needs to extend.
You can tell when the belly pan is working right, the leaves you drive over are blown out the back and not dragged along in the wake.
