This is a tough one, since you'll have no control over what your family does, but I've been struggling with this thought too, so I figured I'd share my thoughts in case you're still here. You may want to consider leaving very detailed instructions for the handling of your funeral in your suicide note, if you plan to have one. Even if you aren't thinking of having a note, you could leave one with only this information without additional explanations should you choose not to tell them why you're CTBing. You could share in this note that it's important to you to have a secular ceremony, including your preferences of poems that could be read (instead of prayers) or nonreligious songs, for example, since they may find comfort in making it seem like a traditional funeral in structure, if not in religious content. You could list who you'd be comfortable speaking at the service and who you'd prefer to stay quiet, if you know who'd spew religious propaganda versus who'd respect your wishes.
If possible, try to address this note or send multiple copies to different people, so that you're not entrusting your remembrance services with just one person, who may choose a religious focus out of their own selfish desires. Hopefully, a group of people will be able to come up with the morally sound decision to keep your funeral secular. They may feel they'd be "haunted" (lol) or at least plagued with guilt if they refuse to respect your final wish. You can even allude to this in the note.
Finally, feel free to make requests over the handling of your body. Someone on this thread mentioned body donation, which is certainly something to look into. But there's also cremation, and requests for where to sprinkle your ashes. I personally love an idea I found once online (for which there is an actual company): you can have a loved one's ashes mixed with fertilizers and seeds to grow a tree. The ashes act as fertilizer themselves, and I really appreciate the thought of having my worthless body finally being put to use for something that can actually contribute to this planet. And I loathe the idea of having my dead body pumped with preservatives and put in a wooden box to be wheeled into a church and then into the ground while everyone who "loved" me watches. Gross!
