I make a similar style of box and have had great results. Here’s a few pointers. Keep in mind I’m not nearly as experienced as some of the guys on here. I’ve made a few hundred boxes and learned a lot in the process.
Side walls are too thick. Try making your grooves in your base 1/8” with a table saw blade set at 5/7 degrees and about 1/16” high. Plane your walls to 1/8”. Just make sure they fit easy cause glue will cause the wood to thicken and you may not be able to fit the side walls in place. Make sure your side walls are the same height cause if not you will only be able to achieve the sound you want out of one side of the box. Heavier wood seems to make a better paddle (bloodwood, Purpleheart, walnut, yellowheart, and ebony are my preferred). Less dense wood works well for the side walls (I prefer black limba, poplar, or cedar). Hope this helps get you in the right direction.
Josh