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the thickness is produced by the proportion of liquid to flour/solids, you can also add panko flakes and tempura for an extra crispy effect and multitextured thickness. not thick enough? add more panko/tempura/flour

Also, the trick you could use to make your life easier and circumvent the problem all together is this: two stage frying, deep par(tially)fry the pieces just so that the batter layer solidifies in place. 1 minute at 350F would be more than enough, probably closer to 40 secs, then finish the pieces in the air fryer.