Will this interfere with the trunk lid or rear bumper when closed?

No. The diffuser mounts beneath the bumper line and does not extend into the trunk seal area. Verify fitment by test-fitting the pieces before final installation and paint.

Should I use adhesive or drill mounting holes?

Both methods are viable. Adhesive keeps the bumper damage-free but requires careful surface prep and full cure time before driving. Drill-through provides permanent security but requires precise hole placement. Choose based on whether you plan to remove the diffuser later.

How stable is this at highway speed?

FRP's rigid construction resists flex and flutter at speed. Ensure all mounting points are secure and that the two pieces are aligned tightly before sealing. Uneven gaps or loose fasteners can cause movement — address these during install.