How about in a coffee can under the bed, counted daily?
Your 17 y/o needs to learn advanced delayed gratification skills now while it is easy to learn and the stakes of failure are low. Even better is if he fails to control his impulses and gets to experience the deprivation and negative feelings of being broke. What did he lose? $2k?
Yes, your 17 y/o is showing advanced maturity, insight, and long term planning skills by wanting to set up barriers between himself and wasting his money. It’s what we adults do when we avoid media and segregate our accounts. Still, in 2-4 years there will be a credit card in his wallet or a cell phone payment option in his pocket. What then?
Make him flex those Mustachian muscles. Put the cash in plain view. Encourage/tease him to waste it on stupid shit and go broke, and make him resist your social pressure so he can stay solvent (resistance? Sounds like fun to most 17 y/o’s!). It’s OK to explain what you’re doing - it can even be a game between you two. Soon he will face rising peer pressure, marketing, offers to go into debt, etc.
In today’s world this is a survival skill you’re teaching. Literally, poor people die years earlier on average ! Be a good mentor and don’t get a CD.