Never built a reloading bench but have built some heavy work benches. One thing to add to all the good suggestions. I have built several out of 2X6 or what ever 2X material is cheapest per sq inch to buy for the top, I then cover it with masonite. It is a cheap way to have a continue surface with no gaps and if it gets beat up or stained add another sheet on top. I just nail it down with drywall nails, but you could use liquid nail so you didn't have all the nail heads.
My current gun bench for cleaning and doing work on my guns is a door that I cut down to the size I needed across two 2 drawer filing cabinets for storage. I then bought cheap carpet runner material at a big box store and covered the bench since it is for clean work. I have a different bench for dirty work.