lock box to hold phone/wallet that attaches to my bicycle I want to bike to the beach, lock my bike up and swim. I am looking for a high security box that either threads through my U-lock, bolts to my bike frame... so I can keep my wallet and phone in it while swimming.
Thoughts?
jg
High Security + Bicycle + Marketable Product is a formula no one has solved.  People keep trying.
A lot of them never manage to launch.

Some seem to have an initial production run then quietly vanish.

Those are just two examples I had literally from memory without delving into google.
Basic problems:

anything you can carry on a bicycle and manufacture for reasonable costs can be defeated in 20 seconds with tin snips or a battery angle grinder.  Moderately 'secure' hardside motorcycle saddle bags cost more than the sort of bicycle you'd generally leave unattended at the beach!
as anyone who has ever had their own lock vandalized or fail and needed to &quot;steal&quot; their own bike can tell you, passersby are totally unfazed by brazen broad daylight bicycle theft and readily accept &quot;it's mine, see some kid jammed the lock to prank me.&quot;  So anything &quot;more secure&quot; than the not-that-hard-to-cut U-Lock shackle is no harder to steal than just the entire bike and now you made it look valuable.
the market for this type of thing is a demographic that has extremely exact specifications in mind for the particular thing that is their preferred solution, and is a bunch of avid DiYers....

The best you can do is a 'keep honest people honest' approach of a small hardsided toolbox or similar that the U-Lock shackle can pass through or the type of combo lockbox realtors use to store keys that shackles to a doorknob that you could pass through the U-Lock.  If you need to leave a phone make it a cheapo android, don't leave both a house key and an ID with the address of that house at the same time, etc etc.