Game, Set, Match

I do not like to waste time looking for mates.   
Here is how I store reusable plastic storage containers...

With the lids attached.

Stacked by shape and size.

Caution:  This method takes up space and does require assigning an entire cupboard for reusable plastic containers!  I am a heavy user, so this solution works for me.

DAILY LEFTOVERS - If your house is like mine, there will inevitably be lidless containers and container-less lids.  Keep these stragglers out of the daily rotation.  Drop mismatched plastic lids and containers into a separate holding area.  The kids can sort through them and find matches as it starts to fill up.  Then...just pop the reunited lids with their container mates and return them to the stack in the cupboard. 

LEFTOVER SOCKS - If your house is like mine, there will inevitably be mate-less socks.  Keep these stragglers out of the daily rotation too!  Drop mismatched socks into a Lonely Sock Box.  The kids can sort through them and find matches as it starts to fill up.  Then...they can roll reunited socks and return them to their rightful owner.  OR have a rolled sockball fight.

Tip:  If a sock is without a mate for over a year, relegate it to dust bunny duty.  (This will not, however, work for plastic lids.)

Absolute Socks

Tween girl's socks may not require matching, as mismatched sox are trending.