not going to find a standard one in that size i dont think. i have succesfully used bigger boxes, cut sections from the middle and then cemented tham back together, sand them a bit and the join is hardly noticable but you need the right cement for the material
as i say the most important thing to how i do it is the right cement for the material, and making clean and accurate cuts and get rid of all burrs and rough edges.
a bit of 1200 grit wet and dry will make the join fairly invisible if you dont want to see it but you can get a good join with careful prep work anyway