You need some easily transportable thing as the basic value. Then you make various derivates and monetary instruments based on that. I think 3 millimeter stainl

You need some easily transportable thing as the basic value. Then you make various derivates and monetary instruments based on that. I think 3 millimeter stainless steel nuts might be the best. They are valuable even in the stone-age future and barter economy, which is certain due to the rapid cognitive decline, which is now quite observable already. Even a layman can test the material and ensure the quality and function of a 3mm nut. Fistful of stainless nuts is 100€ worth easily, and that does not fluctuate or erode, like Bitcoin or Gold.