Following the work presented in the previous post, i routed the chanels for the top planks :

I routed with a 12mm double flute router bit with 5mm passes approximately, with time between passes to let the tool cool down (a makita compact palm router, that i bought for my CNC).

I generated quite an amount of dust and shavings, but hopefully, i have my  home made 3D printed cyclone (link is to hackaday.io project) to help me cope with that :)

At this point, it's a matter of sanding a bit, then applying oil.

I used 8 layers of oil (the oil dries quickly, so i took me a day or two at max), with sanding by hand with 500 grit sandpaper between each layer.

It turned really smooth, and i'm very happy with the result.

The platters have simply been oiled 2-3 times, with little to no sanding in between (they'll be covered by a mat, then by the aquariums, i just wanted to waterproof them a bit)

So next time, we'll install the frame of the second level on top of the first level, secure it, then put the platters in place.