Very hard than you can imagine. Haven't you heard of memory lose before? Brain dimentia and so on? Yes, we all know the danger of writing seed phrases on a tangible materials but cramming it is not the best way neither. Writing on a paper should be out of options completely as it is not secure at all. In my opinion, metal plate has covered me and my seed phrase already.
Look for a sizeable flat metal plate, a nail or needle and hammer to print your seed phrases on the metal by making it in form of hole. It actually takes a little bit time but it is more safer and resistant to both fire and water. Ensure to save it in a dry, clean and private place to avoid hole blockage.
This doesn’t still eliminate the risk entirely because if it is save in a place where no one could actually get to then if there’s is a case of serious brain trauma like amnesia where it is hard to remember past events then it will also make it hard to find where it is stored. So no matter how hard it is to trust people there should be a family member that one needs to share one of the storage places with. As this could also be vital in recovering this funds should the owner dies