Thank-you.
I made myself a build, didn't touch the default kernel configuration, and still ended up with a kernel that had a different hash from the ones opkg will recognize. I figured out how to fox opkg and the kernel modules I can download will work, but it wasn't worth it to me since I wasn't sure what else might be subtly broken.
In the end I used a pre-made snapshot build and was able to accomplish my goal, which was to successfully test the Paragon ntfs3 kernel module backport to kernel 5.10. I was able to use the ntfs3 kernel module I made in my custom build on the kernel created on a released snapshot.
I'll post the procedure and backport patches sepcor for anyone wanting to test it out too.