I figured that as soon as I start working on it again, I will find the old prototype. It didn't work, but I did find the chips I had ordered for it, so I can just make a new one.

 There are two tricky parts to the mechanical assembly, first is the FeatherWing header:

I'm using a feather to get the right spacing and alignment, ad then just soldering the angled headers to the pads. Finished, it looks like this:

I had to cut some of them shorter, where the pull-up resistors for the I2C go. The current version of the PCB has that fixed, as the resistors are moved to the other side of the header.

Next are the pins that connect the antenna to the main board:

It takes some re-heating and aligning them to fit them in their holes, but the end result looks good:

Next comes the chip itself:

Now I need to check the values of all the discrete components in the datasheet and solder them, and it will be ready for testing. That will probably happen in a week from now, because I will be travelling, and I can't take it with me.