I probably found the least expensive hard pen case you can find around - an eyeglass case, the kind with a metal frame, usually covered in some color of fake leather, that snaps open and closed like a clam shell.
I had some around the house, but I have also purchased some at a thrift store where they are usually sold in bags of two or three for 1.49 or 2.00. Out of the three, usually one looks unused, one is disgusting and one has possibilities, maybe.
I can fit three pens in - I made a sort of sleeve from a bit of old sweater to slide them into to keep them from hurting each other, but you can just wrap them in a paper towel. You can also throw in an extra cartridge or two if you want.
One of these might fit in your pants pocket - depends what style of pants. Someone you know may just have one lying around, free for the asking. At any rate, they are easy enough and cheap enough to get that you can put your pens in one while looking for something you like better. If not in your pants, then tossed in a bag they should still be fine this way.