How do you create your own hardware wallet and store your private keys instead of purchasing one?

Seeing all the issues with ledger and trezor lately, it's sparking me to want to make my own hardware wallet rather than purchase one from a company. I assume this is possible seeing as this is what was done in the beginning, but how exactly can you do this? Is there a guide for this somewhere that is reputable?

I just want to hold my BTC and not have to worry about any company possibly updating their way to my passphrase.