Well, if it doesn't turn you on and it's not for personal use, fill it with whatever you have on hand. I don't know what your concern was? You don't need to be an engineer to fill the outside of a tank. Of course, cover the side outlet that is 1 "1/2 well, no matter what you irrigate is the base of the tank.
Regards
Plumbers in the area have told me that when tanks are emptied they often break. It has happened in a plan of houses of the rionegrino state which had all tanks simply buried, filled with earth, and after a few months they were all broken. That's why I don't want to take the risk.
They break because there is external pressure and there is no internal pressure. That's why I told you about making a type of cabinet (apart from being able to take it out). Ask the plumbers in the area how they solved it. Also remember that tank is not a cistern. Greetings Pd: I understand anyway that you already solved it because I see the accepted solution, so it makes little sense to insist on the issue.