An exact-sized box through one of the big shippers is likely your best bet. I often make my own boxes. I save large cardboard sheets from furniture purchases, etc. Making a box isn't hard if you take a minute to understand how it goes together. I measure the item out (in this case a frame) and ensure that the box measures just slightly larger (1/2" or so) than the item's dimensions to accommodate bubble wrap or foam sheeting. The key is to not give any room for shifting so that the item seats snugly in the box with a layer of protection between it and the cardboard. I then do my best to fill any void with air pillows saved from online purchases sent to the house. Obviously making a box isn't the easiest way, but if the paint is worth protecting, it's a safe approach.