What you'll need:
1) A shallow Tupperware box. The multi pack from ikea is the best I've found. £1
2) a multi pack of sponges £1 MAX
3) old dish towel 50p
4) "stay wet" Rowney palette membrane and blotting paper kit. From any good art shop maybe £3 maybe less from Amazon.
Cut the sponges to fit into the base of the Tupperware snugly. Try to give yourself a good even base.
Then cut the jay cloth to do the same. The sponges will absorb the water and the cloth will bring it to the surface.
Once this is done put enough water in to make all the sponge wet but not soaking.
Again cut to fit, your blotting paper (the thick sheet to put on top of the cloth.) and the pallete membrane to go on as your final layer.
First press the blotting paper into the cloth until it's fully wet then lay palette paper on top carefully. You don't want puddles on top of this layer as it will mess up your water to paint ratio.
And there you go! Sealable palette complete! No need to replace the sponges or anything else but the blotting paper once every few months and the palette paper when it's full of old paint because you forget to leave the lid off!