Check out option 3 for cutting a spline slot in this woodmagazine article: https://www.woodmagazine.com/woodwor...oinery/plywood
Since I have no slot cutter, this would be a way I could cut the slot. And given I already have a mortising jig that I could use for this, that could work. I certainly have smaller cutting bits.
I'm still wondering about the rabbet idea, especially since the baseboard will be fixed to the sides and divider. The divider will be screwed and glued from underneath. The sides and back, well I haven't decided yet. I want to avoid screws from the outside on the sides. The back may well get screwed on like the divider.
Given the amount of support the baseboard will get, and that the amount of weight it will support is likely to be a number of porcelain platters, I'm not sure that the join needs to be in the middle like Biagio was suggesting. In which case, how much weight is it likely to have on it, and what does that mean for the style of joint required?
Mostly thinking out loud and wanted to share where I'm at.
If I chose to do a spline, does it have to be A grade ply? Could I use a lower grade ply since it will not be visible? Given the length involved, even if there was a few voids, it shouldn't make a lot of difference you would think?
By rabbet, in this instance it would be a "double rabbet", in that both pieces would get a rabbet to increase surface gluing area.