What can I build to store firewood?

If you mean a container near the fire, any metal handled flat carry all or farm containers would work. For a wood box, my Mom used an old chest she had for years and filled it as she heated her house with wood. If you mean for outside, pallets or anything to keep them off the ground works well.

https://www.hometalk.com/diy/outdoor/sheds/easiest-firewood-rack-the-v-rack-43422533

9 more ideas: https://www.thegardenglove.com/9-super-easy-diy-outdoor-firewood-racks/

A wooden crate or bin can be used. You can also find metal racks that sit next to the fireplace. Check places like Goodwill. A storage bin built into a cabinet next to the fireplace can also be nice - especially if it has an outside access door so you don't need to drag messy wood through the house.

Here are some great diy ideas https://www.thegardenglove.com/9-super-easy-diy-outdoor-firewood-racks/

Here is what we built! https://www.charlestoncrafted.com/building-a-wood-shed-from-old-fence/