Add Length Of Cable To Microphone

We make underwater microphones (hydrophones) with standard stock cable lengths, but many customers need special cable lengths. I would like to have a product option where the customer could select from any of the standard lengths or a "custom length". If the customer selects "custom length" he would then be prompted to enter a quantity for cable and then the cost per quantity would be added to the item price. Does anybody know of a way to do this?

Our current way of handling this is to ask customers to buy cable as a separate item, but it leads to confusion with customers and with our staff when more than one hydrophone is purchased.

If anybody knows of an add-on or technique, or would like to quote the creation of such an add-on, I'll be grateful to know about it.

Out of the box you can only create new product options and specify possible length as variants (e.g. 1m, 2m, ... 100m)

Use default options modifiers functionality to add extra cost to the hydrophone for different cable length