How would I join these two objects together at the specific areas?

https://preview.redd.it/gs1gvsrmqnge1.png?width=3115&format=png&auto=webp&s=f66f58bc3338509df9e72ffb980ae25b73f937ce

Object is going to be given thickness to be part of a 3d model print.

I've been spinning my gears to try to figure this out.

Went into Edit mode with both objects selected and tried to merge edges on one side, with edges from the other by selecting and pressing M and merging at center, and it's not doing so. What should I do to make this one cohesive mesh?