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I just printed back_half_pt02.stl and it appears that half of it is hollow. It took me a little while to figure it out, but it looks like it has 4 walls on one side of it instead of 2, so the infill is being placed in between the tiny space between the two walls on each side, instead of the large empty space in between. If I'm not making sense, just load it up in any slicer and do a layer-by-layer view and you'll see what I mean.
Edit: Just realized that back_half_pt03.stl does the exact same thing where it's meant to connect to part 2. I haven't checked any others yet as I'm printing part 3 right now. Is this an intended part of the design?