An idea often not agreeable to purists...use an extra rad for hot weather and or heavier load travelling. Install aux. rad right behind the grill in the airflow.
I've used a GM pickup heater core on the 1380 powered sedan c/w trailer here in North America and like item in loaded 1098 powered ROO in Australia and North America. There are lots of off the shelf options at auto parts stores (here in Canada) to select dimensions etc. to fit your space. 1380 also had a puller fan in the LF inner guard with on/off switch on dash - simple/effective since I knew when the running conditions were hot and could leave switched on at highway rest stops.
Easily blocked off or removed in cooler weather lighter duty. Likewise an oil cooler can be easily blocked off in cooler weather.

One bolt in apron locates the simple frame to hold the rad

Same aux. rad loaned to Cooper S owner for Spring Thaw Rally (ignore the complex mounting he cobbled together.

Nose on view of the S.

Same rad (borrowed to help) Ex-Mini Miglia race car at Can-Am Mini Challenge race at Mid-Ohio last June...ended overheating.

Same aux. rad back in Aussie Van after Mid-Ohio (photo to show bagged dizzy during extreme wind/rain events.

Same aux. rad out front on Diddy's round the world Van AFTER cracking both block and head on crossing the Mojave Desert to Arizona...we got him home 1,800 miles where engine rebuilt with new block & head.
For what it's worth.
Rick/Hunter2