I've not used root tabs for that exact concern and instead dose liquid ferts into the tank, the plants are doing very well with this method of plant feeding. I use seachem ferts but there are plenty of fert dosing options.
There are a LOT of low humidity plants, many house plants work great as riparium plants. Typically keeping emersed form of aquatic plants (some exceptions being pennywort, ludwigia, and creeping jenny) and some more tropical plants need the higher humidity.
Starter list of easy low humidity and low light plants:
You can find a larger list here, contributed by members of the forum: https://www.plantedtank.net/forums/25...lant-list.html
I've found it more depends on the plants you are keeping than the tank to decide on planter size, some don't stay that small (for example my dwarf palm and peace lily 'dominio' are 24" tall and several inches wide and need a larger container).
Photobucket is not working atm so I can't link photos of the planters I use but they're just simple cheap plastic shower caddies (google "shower caddie riparium planter") I got for $2-$3 at my local grocery store (also check dollar stores).
The black foam "riparium trellis rafts" you are asking about were sold by a member of this forum, hydrophyte, here and at their riparium supply site but they no longer sell them.