It doesn't implement the U1 protocol, rather it's a Bluetooth LE-based tracker that works with any phone, but it fits your form factor. Buy one and peel it apart, or look at their FCC filing photos:
plus the processors, piezo buzzer, antenna, as well as injection-molded plastic shell.
To build this from scratch, you need an electrical engineer, a programmer, a manufacturing engineer, a PCB manufacturer, and an assembly manufacturer. Be aware that hardware startups are hard, you can't just throw up a webserver and scale later, you are probably needing at least 800 hours of skilled engineering effort, probably double that if you're new to electronics manufacturing, just to get a comedy-size PCB conglomeration which would get you tackled by security the airport, might have 3 days of battery life, costs $200 each, and can be found by an iPhone using the Find My network. Then you have to miniaturize it and get the BOM cost under $15 (which won't happen at quantities under 1,000) and the assembly tooling semi-automated so you can build it in under 120 seconds of hands-on operator time and then you can sell it for $29.99, until someone on Aliexpress rips off your design and sells it for $24.99. See https://www.ycombinator.com/library/47-product-advice-for-ha... for some less bitter advice.
https://chipolo.net/en-us/products/chipolo-card-spot?cl=head...
Also, the Tile Slim tracker is very comparable:
https://www.tile.com/product/black-slim?pack-size=1&category...
It doesn't implement the U1 protocol, rather it's a Bluetooth LE-based tracker that works with any phone, but it fits your form factor. Buy one and peel it apart, or look at their FCC filing photos:
https://fccid.io/2ABXLT1601S
https://fccid.io/2ABXLT1601S/Internal-Photos/2ABXLT1601S-Int...
or a teardown:
https://youtu.be/Vl6EdSlNKQo?t=61
It uses flexible PCBs, thin rigid PCBs, and an extremely thin battery like this one:
https://www.ultralifecorporation.com/default.asp?LINKNAME=LI...
plus the processors, piezo buzzer, antenna, as well as injection-molded plastic shell.
To build this from scratch, you need an electrical engineer, a programmer, a manufacturing engineer, a PCB manufacturer, and an assembly manufacturer. Be aware that hardware startups are hard, you can't just throw up a webserver and scale later, you are probably needing at least 800 hours of skilled engineering effort, probably double that if you're new to electronics manufacturing, just to get a comedy-size PCB conglomeration which would get you tackled by security the airport, might have 3 days of battery life, costs $200 each, and can be found by an iPhone using the Find My network. Then you have to miniaturize it and get the BOM cost under $15 (which won't happen at quantities under 1,000) and the assembly tooling semi-automated so you can build it in under 120 seconds of hands-on operator time and then you can sell it for $29.99, until someone on Aliexpress rips off your design and sells it for $24.99. See https://www.ycombinator.com/library/47-product-advice-for-ha... for some less bitter advice.