They will work if covered with plastic so the mold can be removed easily. These pieces look pretty large. Remember a finished piece will be at least an inch and a half taller and 3 inches more across than the mold.
The short piece without bottom could be used to make a planter either inside or outside of it. If you think the planters are too tall, they might be cut shorter with a jig saw. If there is no inside lip on the cut off piece, they too could be used as either inside or outside molds. Whatever you do be sure the finished piece can be removed from the mold. Just plan ahead and don't be afraid to experiment.

Lindasewandsew (I love that), Yes you can do exactly what you said to cover them. After you've encased with screen, do a thin coat let it set up a short time then apply another thin coat. You may want to buy some "Concrete Acrylic Fortifier", comes in about a quart size bottle, to add to your water for both mixes. Helps the two layers stick together. It's usually sold wherever you bought your cement. If you have a lot of Elmer's glue you can use that in place of the acrylic fortifier.
You may also want to work on these upside down. Do your side walls and bottom, then when the two thin layers have setup, turn the planters right side up and finish the top edge. Do you have to do the interior? Remember to keep the mix damp by spritzing with water and to cover with plastic to keep the mix damp so you can keep adding to the piece. Work in the shade. Don't forget your drain hole. Billie

Why leave the metal molds in the finished planters? It seeems like a lot of work, and the molds can never be reused.

Fred, I assumed that they were old planters and instead of throwing them out that Lindasewandsew was going to cover them to recycle, like people cover styrofoam coolers. Billie

Thank you for the advice. These planters showed up on my porch, left by a neighbor who found them in Lowe's parking lot. I hadn't thought of using them as molds, but with the amount of 'stuff' on the curb in southern California on any given day, you can just about make a shopping list and go find it, so if they're covered or encased in cement, they can be planted, then I'll go 'shopping' for something else to cover. Maybe they don't need cement on the insides, hadn't thought about that either. The pots could just rust away inside. You both sound like you've had a lot of experience with this. I can't do them for a few weeks, but am excited about trying it out, and will tackle a small project first with a few plastic bowls to see how it works. Linda