My bunnies love levels in the cage. Korr is almost 6 and he does well jumping up and down. I do have a short distance from the steps of about 6-8 inches, so they aren't jumping a huge height.
For a cage that size, you should be able to get some wood cut to the right length and support it using wooded dowels. You can get planks of wood that range in width, so you can get what you want. You would probably need to drill some holes in it so you can attach it to the dowels, but there are also other things you can use. It would be a good idea to cover the wood with some stick on tiles or something to keep it clean.
I would say to use the NIC grids, but the width of the cage is too big to just use 2. You could do it if you support it well and have something solid on top. For this you just need to zip tie the grids together and zip tie them to the cage. Run the dowels underneath and zip tie the grids to the dowel. Decide what you want to cover it with and attach it.
For a ramp, you can use more grids. You will need to overlap them for strength and you might want to use another dowel to support it. You can them zip tie the ramp to the cage.
