I too am in the process of making new door cards. I am making them out of PVC foam board. You can get it at sign shops. If you're careful, you can make all of them with one sheet. 3mm was around $12/sheet and 6mm was around $20.
3mm was very similar to the fiberboard in thickness and weight. I went with 6mm because I wanted superior sound deadening properties, as this whole project started because I couldn't hear the radio at highway speeds. I am going to glue carpet to the face (again to absorb sound) and foam to the back wherever I can.
I have rough cut them out, but not much else yet. I was thinking about industrial strength velcro to attach them, but I will most likely drill and screw them in to get a better seal to keep out noise.
JW
1st pic: Laying out cards on PVC sheet
2nd: Adding padding under rear cargo mat
3rd: The help