@ciccio, Just some quick remarks:
1) It used to be that brokers accepted investment-grade bonds as collateral for futures. Even if IB doesnt allow this, bonds are a safe asset which you can sell anytime to fund your margin. These days the method is not so useful as german government bond yields -0.6% but hey thats better than -1%. You can probably still find some short term eurozone corparate bonds yielding positive returns, for example Deutsche Bank, available at IB,
https://www.bourse.lu/security/XS0519942139/166686
2) When you think about risk of stocks, bonds, USD, etc, keeping a large sum at IB is also not so great! IB itself has a BBB+ credit rating which is investment grade, but quite low, so this is not risk free by any consideration.
3) If your bank account in your home country does not yet charge you for keeping the money then move it there, and move it back to IB whenever you need a higher balance for margin. This will be cheaper than paying 1% to IB.