The workbench has a top now!
After browsing around Home Depot for a bit, I ended up going with 8 foot long 2×8″ boards for the top. A little rougher than the 1×8″ boards I was initially thinking about, but the 2×8″ boards were significantly less expensive. The end result is essentially the same though. The edges aren’t exactly the most square so there are some tiny gaps between each board, but nothing big enough that stuff will fall through them.
Next task will be to take the sander to it and smooth it out some. After that, it will be ready to use (unless I decide to do some coats of polyurethane or something).