That's not possible to convert your regular USB to a hard wallet to store BTC with full reliability. As Simple or regular USB is just made up of storage unite and a few resistors etc. which have nothing to do with security, encryption, display, network connectivity, etc. etc. as hardware wallets have in them. So if you really want to make a DIY hardware wallet at home, then my suggestion is to read the following threads.

• Open Source Hardware Wallets--> This thread is full of all the details about wallets and it also contains a section of DIY wallets where you can get an idea to make one.
• DIY Bowser Hardware Wallet
• Krux DIY Hardware Wallet
• Make your own Trezor Hardware wallet for under $40
• low-cost hardware wallet for less than 10 dollars--> This might get your attention as it's the cheapest one you will find to make. Note* he just made a plan to make one but AFAIK he did; it made it actually it's just a plan as till now he doesn't talk about any progress.

but i prefer you to not use DIY wallets to store huge amounts of BTC as the parts you will be buying are open-source and when you will buy them from third parties there could be viruses in them. Such as a YouTuber bought one and lost all of his money (he bought it from a third party and get scammed). ( Never, ever, buy your hardware wallet from a third-party)

Plus if you really want to make one, you should have a basic level of knowledge about, encryption, electronics, and IT, as you can make one by watching YT video too which needs no experience.