For the ReRam are we limited to pre-made cells or can we make our own memristor devices

It's better to be asked in reram channel. But I think @Tim Edwards can help you here.

There is so little documentation on the device that I would strongly recommend using the pre-made cell if you want the lowest-risk solution. However, there is no prohibition against experimenting with different geometries.

@Tim Edwards Is this lack of info relates to the SkyWater team?

It relates to the fact that the process was developed by MIT, not by SkyWater. SkyWater has not done ReRAM before, and their commercial PDK doesn't support it.

In efabless are you going to cooperate with those guys to provide better documentation? I'm asking because it seems that the ReRAM is becoming one of the most popular features in the process.