I have found that sorghum or buckwheat mixed 2:1 with any of corn, tapioca, or potato works for almost anything. And that this doesn't have to be carefully measured, as long as the total flour is right. However, the liquid may need to be adjusted - don't add it all first up, and see whether it is needed.

Commercial mixes are either to heavy on rice flour and thus are gritty, or are too heavy on the tapioca/potato, and thus are rubbery.