This is why some form of supranational insurance is needed, for example through the creation of a European debt redemption fund (ERF) financed by dedicated resources of the participating countries. The mechanism can be designed in a way that prevents systematic transfers across countries while reducing the risk of financial instability for the area as a whole.
The introduction of an ERF would strengthen national commitment to debt reduction (as the share of national debt transferred to the fund would be backed by a dedicated revenue stream) and reduce the systemic relevance of (residual) national debt. This would be 2  instrumental in enhancing the credibility of the no-bailout clause and the enforceability of European fiscal rules.