The antonym for "more uncustomary" would be "more customary". A customary action is one that is usual, conventional or traditional. In contrast, an uncustomary action would be one that is unusual, unconventional or not traditional. Using the antonym, "more customary" would describe an action that is even more typical or expected. For example, sitting down to eat a meal with others is a customary action, while standing on the table to eat the same meal is an uncustomary action. Using the antonym, "more customary" would describe simply sitting down to eat, as opposed to any other unconventional behavior.