On UKScrappers another crafter recommended a re-purposed tray from Poundland (originally for father's day) to store PL cards, but my local shop had run out when I got there. Disappointment all round.
Seeking solace I headed to Ikea, and found a couple of 'Tjena' boxes in the sale; reduced as yellow is not everyone's choice of colour. They do sell other colours too at full price, but I went down the bargain route!
To make the boxes strong enough to hold my cards I used both wet and hot glue to put them together...
...and made use of large amouts of washi tape to secure the edges of the lids and add a little decoration.
I planned to use the smaller of the two boxes for 6x4" cards, but it was a little wide and would waste storage space,
so I made a partition (from a cereal box covered in the wide washi tape) to store a set of 3x2" cards too.
The larger box came with 12 partitions large enough for 3x2" cards...
..so I sorted them mainly by colour, using washi tape on top of each partition so I know which colour is in which space. There's also a little room for a few die cuts made from the cards.
The smaller box has all my 6x4" cards sorted by colour, and the space at the side holds 3x2" journalling cards.
So now I can find my PL cards I MUST use them on 12x12 pages and pocket pages too! And as the box is still full, no more buying Project Life cards for me :)