On the west coast, Incense Cedar is often sold as very cheap fence board. Libocedrus decurrens is aromatic and dimensionally stable. Incense cedar gets a pocket rot that leaves holes in the material, but many boards are defect free and can be selected from the stack. Fenceboards are frequently available as in (actual) 5/8"th by 7-1/4" dimension. These can be ripped to produce the 6-5/8th Medium. The 5/8th dimension is smaller than the 3/4th thickness most hive plans are designed around. In previous decades you could get rough-sawn incense cedar with a nominal 7/8th or greater. That had the opposite dimension problem.
Fence boards can be frequently located on loss leader at 1.50/pc. That contrasts favorably with the high cost of #2 (or better) pine.
I use cabinet making biscuits (size 10 or 20) to join butt jointed pieces. I use Titebond II or III and galvinized drywall screws in the build-up. I compensate for the undersize by cutting the 19 7/8"th long side just a little short. I don't seem to burr on the slightly greater than nominal end gap, and the sidewall frames fill well. In full dimension boxes, the sides of the 1st and 10th frames facing the walls frequently were left undrawn, the slight oversize seems to help and avoids the burring and bridging that happens with 9 frame spacing.
The butt joint join makes using a 6 foot fence board possible as you gain 1 1/2 inch on each end piece. total dimension required is (2)x19-7/8 + (2) 14-3/4=69-1/2" You actually have enough length to use "dog ear" pattern boards.
I am now into my 5th year with biscuit joined boxes, and no breakage. I square the boxes with a carefully cut diagonal stick during glue up. The screws are mainly to avoid need for clamping and give a little extra tightness to the rabbet top area.