Just curious about the "serious Rf problem" on the FD cape. I took a peek on the website and most certainly it looks to me that their enclosure is not optimal when it comes to cooling the raspi cpu. 71 deg is pretty close to the point where the raspi will start throttling down the CPU. I am running one of my rpi 4 at 2Ghz at 50 - 60 deg, but with lots of cooling and a big heat sink. On the other hand my rpi 400 is running at 2.2 Ghz as is and it seldom gets over 60 deg.
One solution is to put an extension spacer between the FD and the raspi so you could fit in a larger heat sink and design the enclosure with the fan mounted on the side so that you put he majority of the airflow between the cape and raspi.
Well, it's an easy fix, but first what are the RF problems?