An alternative that works for simple cases (and covers 95% of my alerts) is using alert2.
This uses Validator for the validation, which uses Popovers from Anvil Extras.
You can clone both Validator and InputBox, make sure Validator depends from the third party Anvil Extras, make sure that InputBox depends from Validator and add InputBox as a dependency of your app. You don’t even need to add Validator or Anvil Extras.
This will not cover complex forms, but will allow you to use fairly complex input forms with validation without even creating the forms! This is how I manage all the input alerts in all my production apps.