I might replace the basket with a piece of 3" or 4" PVC drain pipe with several 1/4" or 3/8" holes drilled. This will allow you to use just about anything to fill the wicking column (PVC Pipe). Some ideas that had hit me were: old pantyhose, old athletic sock, ANY kind of fabric, sponges, and those pellets that expand when wet. With this (whatever) full of mix (and packed in), you don't get the water reservoir full of growing mix. The sock/stocking/etc acts as a filter of sorts. You can even put this over the outside (waterside) of the pipe and hold it in place with a couple of zip ties. This will give you a more linear column of wicking material (mix).
Also, use of the PVC pipe would allow you to easily adjust the depth of the water reservoir to the depth you want it. Otherwise, the basket size would be driving this dimension.
Another idea that floated thru my overactive mind was to build the decorative cosmetic outside as separately as possible, only "attaching" it to the planting assembly. That way, you can replace the "insides" and keep the outside look without a total rebuild.
If you do any caulking of any seams, use the type that seals gutters as it is meant to be under water after it cures. Regular caulk for windows and doors will not last under constant flooding conditions.
The water-input pipe/hole and the hole you cut to allow overflow of the water reservoir should have some screen material over them to prevent mosquitoes from using the reservior as a hatchery.
One last thing, the size of your final assembly will drive the need to have it be level. The wider you make this thing, the more critical the leveling and the more tendency the wood will have to sag under the weight (remember, they get wood wet to bend it). I had even thought about putting a couple of bubble levels on the edges for both axis.
Hope some of these comments will help you make good decisions on your design.
Well, I've talked myself into building one of these things.
Dang, another project.
Ted