Open value attributes? Letting the customer enter any value?

Is it possible to create a product attribute that lets the user enter a number?

I’m trying to set up a store that will sell cables by length, and I want the customer to be able to determine how long they want cables. On top of that, the customer will need to determine how many (quantity) they want, so overriding that isn’t an option. And since every customer is different, and we charge by the foot, we don’t want to force people to buy 1, 5, 10, 20, 50, 100, etc. feet if they only need 60.

I’d love to have a product’s attribute titled “length” that is just a box for the user to fill in with any number, then I can multiply that by the quantity of cables they want, figure out the weight behind the scenes, and give them a price. But product attributes seem to require a pre-defined list of possible options. Suggestions?