Simple spool holder. The OpenBeam corner pieces were a bit overkill for the bottom of the base. Also, the screws would make direct contact with whatever surface it's on. The printed corner pieces have a raised foot at each corner that protrudes past the screws.
The rod is a piece of 8mm linear motion rod. The clamp piece is bolted together with M3 x 18mm bolts in a crisscross pattern. Each side has two small holes to be tapped and two oversized through holes. The clamp is designed like bicycle/motorcycle handlebar clamps. Tighten the lower bolts first. The upper edge will have a gap. Tighten upper bolt to squeeze the rod.