Hi, I think I'd want a pretty strong box, which means box or dovetail joints, particularly for the donation boxes.
Donation Boxes: These would depend on where they are located and whether those locations are open. Some churches keep their sanctuaries open, unattended, which leaves the donation boxes vulnerable to breakins and even theft. So these probably need to be bolted down. One alternative would be to build a box with a built in stand.
The drop slot in the box would drop the contribution through what looks like the box, down into the base. The front or rear of the base would be heavily weighted, but would have a concealed and locked door for access. I would make the stand narrower than the box to give the illusion it is merely a stand.
The top box whould likely have a slanted lid with a visible lock on it, even a padlock. That would give the impression that the upper box had all the goodies when in fact, the bulk of the contributions would remain out of sight in the base.
I guess that suggests a little cinicism about humanity, but I confess to that flaw.
The suggestion box just needs a wide slot on the top, and I would suggest some kind of slots on the side or underneath, to hold paper and pencils for the suggestions. I would put a lock on it, mainly to communicate that it is private and anonymous. I'd put a small padlock there so when the key gets lost, you can easily cut it off. It will get lost.
Just my thoughts. I think oak is a good choice, white if you can get it. Scrape, don't sand. Finish appropriate to the rest of the furnishings, the pews in particular. You could use ply for the panels. These are likely to be there for a couple of generations, so build solid.
