I fitted neoprene side panels inside my cowl to stop air passing either side of the radiator. That's a simple mod using polyethylene (high density) neoprene.
The front of the Trad is messy, and fitting an under-tray between the cross-head and engine was an early mod. All that takes is a piece of sheet aluminium (cut to shape). You don't need to drill the chassis because it can be fixed securely with overlapping penny washers.
When I ran my Roadster with a front spoiler, the ducted air was directed onto the bottom radiator tank, and the under-tray kept it in the engine bay until it could spill out around the exhausts.
At that time I didn't have the confidence to make a curved front valance. Once I could I removed the spoiler, but there's no obvious difference in cooling.
Cain has been experimenting with a super undertray around his engine, and talking to him it seems he's found a big difference.
So anything you can do to tidy up the messy airflow has a good chance of reducing temperature, and might even reduce drag!
DaveW '05 Red Roadster S1 '16 Yellow (Not the only) Narrow AR GDI Plus 4
