TOPIC: integrate a dtdonate type "user defined amount" field

I would like to be able to include an optional donation field on the registration page where the registrant can define the amount they contribute.

Right now I have a custom field with set amounts in a dropdown on the registration page and a separate dtDonate page with a user defined amount field. But most people are only willing to take the time to complete one form and the increments in the drop-down may not necessarily suit their budget.