Zen Cart price by area / JS modification

I am currently building a site for a client using Zen Cart, the site will be selling printed banners.
I need some help to develop the pricing system so that a customer can enter the dimensions of the banner required (e.g. 3m x 2m) and Zen Cart will calculate the price.
The price per square metre will be the products base price as setup in the product admin section of Zen Cart, so a banner with a base price of £20 will calculate as 3m x 2m = 6 sqm x £20 = £120
This needs to be done using JS so the price updates live on the product page as the customer enters the size values.

Currently I have live pricing updating via JS using the inbuilt Zen Cart attributes system but this only allows for an additional price per attribute and not an area calculation.

I also need a way of customers to be able to enter measurements in feet as well as metres - open to suggestions on the best way to implement this.

In the future the client may look at a live banner customization system if the site does well, this would involve editing the banners live on the screen (adding images and text etc) but this is further down the development road. Please bare in mind that the winner of this bid would be contacted in the future to quote for this should it be required.

I cannot give the URL on the message board to the current site but am happy to give it to anyone who bids via the private message system.

Cost is key here people - as is the ability to do the job in a timely manner.

Freelancer to be chosen very quickly (this weekend) and work to start next week if possible.