SeedSigner is no doubt the best FOSS DIY bitcoin project for a signing device (aka hardware wallet). Simply put, if you make habit of usage with this device, you'll learn first hand the interaction with bitcoin's timechain, that is, managing keys, generating wallets, and most importantly, recovering funds in case of emergencies.
Takes less than 100 bucks to gather the pieces; can set a good weekend to build, try, and even show it to others and take of note what they think.