Cash in an envelope without a return address? Extra paranoid drop it in a city/neighborhood mail box collection point rather than from your home. Just make clear on the inside that it's a donation.
Other than that there's crypto with CERTAIN currencies IF they accept it and IF you use non-KYC means of acquiring it which is itself a whole pain in the ass and journey.
There's also buying a gift credit card for cash at a grocery some distance from your home that you don't normally frequent, registering it under a false name and using that though you'll be paying $5-$6 to the CC company as a fee for the privilege and some services may not accept those.
No one knows. Not even they themselves know. It's being actively developed and the policy and envelope actively and steadily pushed forward further and further. What the situation is now probably won't be what the situation is in 12 months.
The thing is as a US citizen they cannot deny you re-entry to the country (if you're non-white I wouldn't be positive they wouldn't try in the future to some amount of such people to detain and possibly deport them or something but if you're a citizen by birth at least that's a lot more difficult even then). They cannot hold you without charging you for more than I think 48-72 hours, one of those though I think unfortunately that might be guidance within the agency rather than law so they could possibly in future push and stretch that period out but at that point we may be through the looking glass entirely. They can take your stuff and resisting them by refusing to unlock your phone will be noted and probably result in future searches and enhanced scrutiny but then again may not.
In most cases in the past (things are changing obviously so who knows where this goes) refusing to unlock your phone as a citizen resulted in detention for a few hours which was considered punishment enough as by then you'd miss your flight and have lost hundreds or thousands of dollars. There were exceptions of course like whistleblower journalists, genuine criminals like pedophiles, etc.
If it's just those airport security TSA people they probably won't do a thorough search and most likely will just briefly look through your photos and apps including messenger apps and emails for maybe 5 minutes looking for super obvious crimes in front of you BUT if they take it to another room or its CBP or ICE they might try and use a graykey or other hacking device on it to download the contents or even in theory install malware as well. You have no way of knowing what direction the search will go once you hand over your unlocked phone so that's the problem and you can't exactly demand it back. They could start out searching it in front of you casually and take it out of sight to a back room with a hacking device and you'd be powerless to do anything about it.
One trick that MIGHT work (be careful, lying to these people could be a crime, I am not a lawyer, check with a lawyer) is saying it's a business phone. If your trip is for business this holds more weight and in most cases they'll give up because companies have rights to keep secrets that the government respects a lot more than the rights of their disposable proles. This works better for laptops as phones they might still assume you can unlock in order to use it. This won't work if you're specifically targeted.
Make a plan before you travel. Are you okay resisting them and accepting the consequences? How far will you resist? Will you really accept missing your flight? Will you accept missing your flight and being detained for 48 hours? Will you accept that plus them confiscating your phone? If you decide you might bend the knee if detained then decide what you need to do to minimize harm like deleting all your social media and chat apps off your phone BEFORE traveling (to reinstall on the other side) so that you're comfortable being able to unlock your phone and hand it over. Be warned some encrypted chat apps won't back-up their messages, look into the specifics of each one. The best thing to do is of course erase your phone before traveling and restore on the other side or when you get back home from back-ups. Have an explanation ready for why your phone is blank though, you had a problem with it for example and had to reset it. Try and create some activity on it so it looks a little lived in.