Cutting a large L shape piece of scrap material to place on your board against the pop-up pins for use as an X/Y Offset reference in the optimizer will be one good way to line of your flipped parts:
The above gray section will be a piece of material you can keep in your shop for future use.
In the Optimizer, place the part with a flipside operation on your sheet, check "Create Individual Flipped Part Program" when generating your GCode, and use an offset X & Y of 12. Then physically place the flipped part on your board against the L shaped material. Keep in mind, the gray part is NOT going to be in the optimizer. It is physically on your board, and the optimizer will calculate it using the X/Y Offset.
This method will give you a point of reference and a buffer zone to more easily line up your part.
NOTE: This is only one example of a way to do this. You could also drill small holes into your spoiler board and place dowels to butt the part against, or you could draw directly on the board to have a point of reference when placing your part.