Last year I stumbled across these cupcake carriers at the $ store. I said to myself HMM!! those look like they would hold 9-3" round pots, so I bought 1 for a buck (big spender). Took it home, dug out my 3" pots and lo and behold the pots fit when I put the top on. Since I make my own labels out of old vinyl venetian blinds the short ones that were made for the 3" pots fit under the lid with room to spare. The next day went back to the $ store and bought 2 dozen more.
These make great seed starter containers. Fill it with| pots filled with your media of choice, put 1" of suitable water in the bottom, when the media is moist add seeds, install cover and put into your growing area in a bright spot (not direct sun), patiently wait - checking weekly until the seeds sprout. Once there are signs of life the cover can be removed - it can be stored under the tray of pots so you can find it when you want to fill it again with new pots and start the whole process over again.
The built in handle in the lid makes moving these around even when full of pots and water easy, just make sure all 4 of the lid clips are correctly engaged.
I go ahead and fill 6 or 8 of these ahead of time so they are always ready whenever I get a new plant or seeds
that need potting up.
If you are unable to find them at your local $ store I have seen them on fleabay for anywhere from $3.95 to $29.99 with FREE SHIPPING each.