If you have upgraded your cell phone then it's better to place your wallet on old cell phone which is no more required and keep it offline. This way you can have an air gaped device with zero investment.
You would have to know exactly what you're doing before going with a DIY approach of an airgapped device. This includes generating your seedphrase completely offline and never letting your private keys online even when signing transactions.
If you know what you're doing, it's a good approach, but definitely not 'zero' investments. You are dedicating a device entirely for your storage, and hardware wallets are not comparatively more expensive than cheap mobile devices.