Why make what you can buy! building a two piece mold to pull a female off the existing bumper and then another layup for the bumper its self, i don't see it being cost and time effective, unless it required to be a custom part that you cannot purchase.
After the OP finishes his bumper, he will agree with you.
And there is a another aspect, taking money out of the pockets of the fiberglass shops, just means they will shy further away from the oddball parts, and its a viscous downward cycle. I don't think they are overcharging, UPS maybe.
