Just another thought. Sequence can be important. Remember to cut the dados or rabbets for the bottom before you glue the box sides together. As long as the bottom (or top) of the box is flat so it will fit in your jig, your can add splines anytime.
Since you're planning on a drop in lid, I'd make the box first. Then, using the actual box, design your lid. I'd make the lid slightly larger than the outside of the box, all four sides. That will allow you to trim it to fit very precisely. Hopefully your lid design will have a flat side (doesn't matter if it's inside the lid or outside the lid. That flat side will allow you to run the lid along your fence on your saw to cut the rabbets so your lid will drop in smoothly without a lot of wiggle. If you have one, can also use a router table for that step. Sneak up on it, make a cut, test fit, sand a little, if necessary trim a tiny bit more, etc until you get the fit you like.
Since you're planning on a drop in lid, I'd make the box first. Then, using the actual box, design your lid. I'd make the lid slightly larger than the outside of the box, all four sides. That will allow you to trim it to fit very precisely. Hopefully your lid design will have a flat side (doesn't matter if it's inside the lid or outside the lid. That flat side will allow you to run the lid along your fence on your saw to cut the rabbets so your lid will drop in smoothly without a lot of wiggle. If you have one, can also use a router table for that step. Sneak up on it, make a cut, test fit, sand a little, if necessary trim a tiny bit more, etc until you get the fit you like.
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