My thoughts on a pre-made shelf is fearing that the brackets for the premade shelf would be unlikely to coincidentally attach to the 2×4s on the other side of the sheet rock. I fear that the weight of the monitor might be more than the sheet rock will hold up and the hold thing will come down. If I'm wrong, I'll just buy one of the shelves. If I'm right, I'd prefer purchasing two pieces of wood, have it cut to size and finished. Then one 24 inch piece would be attached to the wall and screwed into the 2×4s. The brackets can be attached to that and the the second 24 inch would be attached to the brackets.