Seen a few people around asking for a solution for where to put the pen, I think I’ve found a workable solution until someone develops something proper. You can buy adhesive pen loops for cheap (specifically I got these ones to colourmatch the pink) which can then be cut down and stuck onto the inside of the top cover of an expansion card. You will need to remove the padding foam on top of the port, and cut a little notch in the sheet metal casing to let the elastic poke through. I used a USB-A card, because it’s one I’ll always have in the laptop and it’s cheapest to replace if I messed up, but this is probably easiest done with a wide connector such as SD or HDMI.
(Please excuse the awful job I did keeping the metal intact, I seem to have misplaced all my pliers to bend it back into shape)