The cheapest and easiest DIY hardware wallet is build with 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. You can make it literally in few minutes. The firmware is open source and you can flash it online with Chrome/Edge web browser or compile it yourself.
LNbits can be used with Legend OnchainWallet extension (https://legend.lnbits.com/) , but other wallet applications can be used. You can flash the 0.3 version firmaware on https://lnbits.github.io/hardware-wallet/installer/
You can print 3D plastic cases. This is one I like:
This is the GitHub page of the project: https://github.com/lnbits/hardware-wallet
This is a video in which you can find Ben Arc’s explanation.
You can also flash Jade Blockstream Firmware on this device, below you can find a step by step video tutorial.