I proposed to the Cardano Foundation to have a look at the AirGap Wallet/Vault (https://airgap.it/) and they referred me to the forum. So here I am.
In my opinion this open-source software is a very promising and sustainable alternative to hardware wallets. Unfortunately, it does not support Cardano yet. When being asked, one of the members of the very responsive development team (https://t.me/AirGap) told me that they would be interested in integrating support for Cardano but
rely on support from a foundation or company behind the currency
Apparently, this already worked out well for a range of other cryptocurrencies (e.g. Polkadot). I think that the integration of Cardano into AirGap would strengthen both communities and would further democratize Cardano by offering a cheap yet secure solution for people to store their ADA. €60 for a hardware wallet is quite a lot of money for many people, but old smartphones are readily available in heaps.
When being asked whether they need funding, help with the implementation or both, I was told:
It’s a little bit of both. Integrating a new protocol is a lot of work. First, the initial integration has to be done. Then everything has to be tested and the code has to be reviewed by a someone, which is quite costly. And last but not least, we need to provide maintenance. If there is a breaking update in the network, we need to react quickly. We also need to make sure that there is always a node running that we can connect to, which can be a lot of work as well.
I’d be interested to hear your thoughts.
Disclaimer: I am not affiliated with AirGap, except that I am an active user, who would like to bring his ADA into a safer haven.