This is probably one of the cheapest and easiest DIY hardware wallet, and you can make it literally in few minutes including loading firmware!
It's not the safest hardware wallet around, but firmware is open source and you can flash it online with Chrome/Edge web browser or compile it yourself.
Only thing you need to buy is TTGO T-Display ESP32 device that cost less than $10, but it should work with all ESP32 devices like M5Stick, M5Stack Core2, ATOM Lite ESP32, etc.
LNbits can be used with Legend OnchainWallet extension, but other wallet applications can be used.
Since I had ESP32 device for long time collecting dust so I tried installing firmware but connection was not working so I had to install few drivers CH341SER, CH343SER and CP210.
After few minutes everything went smooth and firmware was flashed in few minutes, and I was able to connect device with Legend LNbits OnchainWallet.
I already 3d printed two plastic enclosures, so I now I have one more device I can use and they even support Taproot addresses.
https://lnbits.github.io/hardware-wallet/installer/Note that you have to enable Onchain Wallet in extensions of Legend online wallet, and than open it before connecting with device.
This wallet is great for Lightning Network, LNURL, SplitPayments, SatsPay Server, etc.
Legend LNbits OnchainWallet:
https://legend.lnbits.com/TTGO T-Display 3d cases STL files:
https://www.thingiverse.com/thing:4501444