best solution would be creating Double X-Memory modules : Take two X-Memory modules, open them and place both internal pcb´s in one housing. Then you have the second module shell available for your own custom module application....
Some details about the internal wiring of Double X-Memory modules you find in the HP-Museum article folder :-)
Christoph is totally right. But for making custom modules it is better to get yourself a Clonix or MLDL2000. These gadgets allow you to put your own custom code (plus other modules as well) inside the HP41. Clonix comes with a module, the MLDL2000 comes with a cardreader shell. Clonix may be sold out (please check with Diego Diaz) and MLDL2000 is in production right now.
I cannot sell or give away the CR shell without the MLDL2000. Please send me an email and I can get you in contact with the source. You can find my address on the MLDL2000 website.