I agree with the professor that for a top 5″ thick, most any material will work. I decided on laminated veneer lumber (LVL). You can think of it as plywood beams, except the the grain in the veneers all run the same direction. It is extremely expensive. I went to Craig’s List and found a lumber yard that sells surplus lumber. They have stacks of everything. I found a 40 ft 3.5″ by 5.25″ LVL beam for only $100. I had it cut into 6, 74″ pieces. It is dirty and weathered, as you can see, but I can see the beauty inside. Nothing an afternoon on the joiner and planer won’t fix
Total is $100.