I made my second level quite small to leave room for the other accessories such as hanging litter pans and hammock that I had for the cage. I simply cut a section of the shelving the same width as the cage -- the shelf is basically 16 inches square. I attached the shelf using zip ties. Attach a piece of plastic canvas to the top of the shelf, also using zip ties (the wire spacing is way too wide to just use a shelf as a surface).
I often add a piece of cardboard to the second level. My rats do not seem to like to urinate in their litter boxes and sometimes urinate on this level. The plastic canvas is good because the urine doesn't pool on the second level (thought to be a risk factor for bumblefoot), but then everything below the shelf gets showered in rat urine. The cardboard seems to be a decent compromise as it absorbs some of the urine and keeps it from pooling or falling to the first level, and it dries pretty fast. It needs to be changed often though (either because it is is smelly or the boys have shredded it!).
Solid shelves are problematic because of the risk of urine puddles, unless your rats are well litter trained or they will let you line the shelves with something absorbent (without chewing or moving it).
I used to have a bird ladder for access to the second level shelf, but my rats prefer to just climb the sides of the cage so the ladder is no longer in the cage. I place the hammock next to the shelf, and on the opposite side of the cage from the shelf I have a large hanging corner litter pan. This kind of makes a continuous second level consisting of the shelf, the hammock and the litter pan.