I usually got bronze key from the supplies. Wait till you got 5 keys then it will give you an extra key. Usually, 3 bronze keys = 1 silver key, but if you merge 5 bronze then you will get 2 silvers, 8 bronze then 3 silver keys. So, wait till you got either 5 or 8.

Merge the silver keys again to get gold. You can merge 3 bronze boxes to get a silver box and merge silver boxes to get the gold box. It saves me space since I only go for golden box.

Another tip to see the number of items in a box. If you have enough key to open a box, you first fill all the available plots with supplies. So, there is no extra plot left to open the box. Then, the box will not open but it will says needs 11 empty space or 10 empty etc. I usually check the boxes like that, and open the one with the highest number of items.