[Dazza335iC]

Yes I've searched...

I'm wondering if there are any off-the-shelf options for covering the gaping hole that's left under the trunk floor after removing the DEF tanks. Since the heat shield cannot be reinstalled on the factory mounting points, there's a huge volume there (even worse if, like me, you've removed your rear mufflers) and it's pretty horrible for aerodynamics. I'm not in the mood to custom fab something at the moment.

[Yozh]

Besides factory 328/335i heat shields to reduce road noise, you are looking to fab something up.

[apexit4]

You can always put in the storage tray from a 335i. You'd gain storage space in the trunk and have a few mounting points for the heat shield. The tray is a simple swap, its just held down with 10 or so bolts. from the inside of the trunk. I did precisely this for the added storage space and mounting points.

[BB_cuda]

I cut my heat shield in half front to back. I had to use a rubber mallet to shape it more deeply upward. The bottom of the flat 335D deck has 2 threadings. I used large washers and shorter screws. I was able to support the shield pieces by the outboard stock screws and an inboard support at each threading. The inboard attachments are covered as I have a compact spare stowed up in there. Hole is not perfectly filled but no longer a gaping hole either. I still get 42 mpg going 70 if roads flat. So, my aero losses aren’t terrible. I’ve seen people close out with a homemade flat close out with side fins. Check out a thread TDIwyse wrote quite a while back on the close out. I like having my trunk available for luggage on a road trip yet have spare underneath.
You have lots of options. The 335i under stowage is not that big. The 328 stowage is more sizable but the 335d exhaust won’t clear the 328 undertray. If you went a single straight pipe to left side, now your in business with the 328 piece.
Most here won’t give up the dual tips though. What a sleeper if you had the 328 single hole rear bumper and all? Haha