Capturing variable packaging (boxing) costs in manufacturing

I work for a small manufacturing company. We manufacture ceiling tiles to order, and sell everything by individual SKUs, so if a customer orders 1 tile on an order and another customer orders 40 tiles on an order, those orders will both go into the same sized box.
I’m trying to figure out the best way to track the cost the finished good in ERPNext (we want the cost of packaging to factor into the cost of the finished good) and how to issue boxes out of inventory.
Since the boxing is variable, I can’t add the box to the BOM.
My initial thought is to run a separate stock issuance for the used boxes and add the cost of the boxes as an additional cost on the work order.
But it would be nice if there were a way to add additional items to a work order that aren’t on the BOM for this purpose. Like if I could add 3 six inch boxes and 1 two inch box to a work order and those costs could be automatically added to the cost of the finished good, as well as issuing those boxes out of inventory.
Is there some way of achieving similar that I haven’t thought of?