First, make sure that the config is not using a pre-configured shape by setting the option menu_shape in the config to CUSTOM. This allows you to set custom slots for days in the rewards menu.
Now, head to the rewards.yml and add the following for every day (check the config.yml to see how many days you have to set). For every day, add day-DAYNUMBER: and replace DAYNUMBER with the number of the day. Then under that section you just created, add slot: SLOTNUMBER where SLOTNUMBER is the number of the slot starting at 0.
Once you've done all that, make sure to save all files and reload the plugin by using /daily reload. Check your new, customized menu by performing /daily in-game. If you see all days, then you successfully created your own shape!