DIY lens filters are so easy to make you might not buy a professional lens filter ever again.
Start with some transparent folders that your kid brother has in his backpack. Cut a small circle from each color. Use a hole puncher and punch a hole near the edge of each of the circles and put them all on a keyring.
Now you’ve got an assortment of colored lens filters that you can take anywhere you go. If you want to add some additional interest to your DIY filters, try creasing them or splattering a tiny bit of water or oil on them for extra effects.