Setup with custom build firmware

Hello there!

I'm thinking through a way to use LEDE with FastPath patches on Archer C7 v2.

There are existing builds by https://github.com/infinitnet/lede-ar71xx-optimized-archer-c7-v2 but I need some kernel modules that I can't install from standard repos due to kernel package version mismatch.

I figured I could (in theory) build both kernel and modules I need (only wireguard so far) on my own.
There's a problem however: the device has limited disk space, so I can't build and ship all modules with the image. Archer C7 v2 however has a USB drive.
Is there a way to build an image for this router model that I could write on the USB drive and then boot from?
Alternatively, maybe, a way to build all kernel modules as packages and put them on the USB drive so that they can be used from LEDE at runtime (probably to a separate partition on the same USB drive)?

This should help:
https://lede-project.org/docs/user-guide/extroot_configuration
https://lede-project.org/faq/before_installation#which_packages_can_i_safely_remove_to_save_space
https://lede-project.org/faq/after_installation#no_space_left_on_device
https://lede-project.org/docs/user-guide/opkg#non-standard_installation_destinations

Great! Thank you, seems now I have everything I need.
By the way, I'm going to use docker to produce images. Are there any existing projects that provide this tooling packaged in docker? If not, I can share wht I come up with.