The next step is to build the boxes.
A few weeks ago I picked up one of these handy Kreg R3 pocket hole tools. They are pretty slick. Here the link:
https://www.amazon.com/gp/product/B000J43A7W/ref=oh_aui_detailpage_o05_s00?ie=UTF8&psc=1After properly fitting to a work-piece like this...
You use the supplied drill bit and are left with pocket holes like this...
I used the pocket holes and wood-glue to make three wooden drawers that are 36", 28", and 16" long by 13" wide or just wider than the water tank once you add 2 x 3/4" for the slides.
I started by cutting all the pieces on a table-saw at the same time, so I could get uniformity of length. This is all cut from 1/2" plywood salvaged from my previous build. The Kreg tool works down to 1/2" width material. Here are all the cut pieces for 3 drawers.
The medium and small box with wood-glue drying...
All three boxes in need of some sanding.
We had a bathroom renovation done a few years ago and they really made the bathroom drawers look nice by edging each of the interior joints with caulk before painting, so this will be the next step. Be sure to select a caulk that specifically says "paintable." I've read a few horror stories of someone finishing a job with silicon caulk only to realize they have to scrape it all off because it won't take paint.
I sanded the boxes last night to remove any rough parts and after a wipe-down they'll be ready for caulk.