Hector,
What you have built is what we throw away and/or burn !(but generally pull apart and build things with!) Its a crate used to transport/import fireplaces. I am sure if you approach your nearest fireplace showroom, they will give you a selection of of these. They come to accomodate 42", 48" or 54" mantels, so are slightly larger. Generally made of corsican pine , but would last 3/4 years. If you were 300 miles further south, your supply would be assured!
Send PM if you want to collect from GU1