Mix 160g rice flour, butter, icing sugar, potato starch and tapioca flour together in a bowl until crumbly and evenly combined; press the dough into a 23 x 33cm baking tray or ceramic dish.
Bake in the preheated oven for about 18-20 minutes.
Mix rhubarb, caster sugar, eggs, 55g rice flour and salt together in a bowl; spoon over the base.
Bake in the oven until topping is bubbling, about 35 minutes.
Potato starch vs flour
Note that potato starch is not the same as potato flour, though sometimes potato starch is erroneously labelled as potato flour. Make sure what you buy is a refined, white powder, as true potato flour is made from the entire potato, including the skin, and is less refined.