order amount increments

Hi all, i have a question and hope you can help me.

I sell products but want to force the customer to buy in boxes of 12. So 12 or 24 or 36 products, etc. Any combination of products is possible, so minimal product quantity is 1, but minimum order quantity is 12 products.

I have the mix max quantities plugin, but this only lets me set the minimum order to 12 products, but doesnt handle the increment of 12.

Any body a suggestion on how to handle this?