Bob,
You need an original lower horizontal panel stiffening bracket. They are made of thicker steel to align with the trunk lid. Repro lower panels have a cheesy stiffening bracket made of thin sheetmetal that isn't formed correctly and even it it was, it is too thin to hold the correct position. I had the same problem on my 28 coupe until I found and installed an original lower panel with the original reinforcing bracket in it. The original panel/brackets made all the difference in the world! I am attaching an original 30/31 panel below the trunk with the horizontal bracket that is correct. 28/29 roadsters use the same brackets as 30/31 do. It also has the vertical spare tire support bracket. You don't need the vertical support bracket since you aren't going to mount a spare tire on the rear.
Rusty Nelson