The idea is to have 1:1 between connectors and D-SUB9, but for testing I just connected all three on the same cable with a D-SUB9 for Laptop connection. This is a stanadard 10x flat cable. Pin 10 is shield if needed. I have not connected 5V and GND on D-Sub9, but I will in the next version. Some people are fanatic about having Gnd because they do not have proper galvanic isolation. But, also 5V is usable in case you have a CAN device that needs to be powered through this cable. The 5V PSU supply 2A and the main board uses ca 200mA, so we have plenty to share within reason.
Starting testing I only switch on 120Ohm on one port at top and I start with a decent low bitrate – lets get basic CAN working before we start testing higher speeds and FD. These CAN FD ports should be capable of 12Mbps, but lets test that before we start braging 
The next piccture is an early mock-up of a possible extra CAN port. You connect this to module1,2 or 3 and you get an extra port. The box have room for these on the other side so the board will be upside down. The height is to fit into the box that will easely fit 3 of these. I already have M12 modules that can be tested, but to develop with that mistake of a SWD I used is hard. Also on the module below is a new, smaller SPI Flash – I am getting braver and starting to use components with connection on the sides – actually found them very easy to hand-solder to my surprice. The MCU in this case is STM32H503CB due to it’s low cost and support of CAN.
For 3 extra CAN it would be better with a scaled down version of the main board with connecting Link to Module. I was thinking of just not mounting the PSU, USB, SD-Card and Ethernet and use the same board, but it might be that a smaller board designet to stand upside down is better – lets see. One argument is that I hesitate starting on a 2nd MCU right now.
Needless to say – this module board can add anything we want – extra CAN port is only an example. Digital IO, Serial ports, analogue IO etc – you name it.