I can't imagine not having a top. Cheap. Easy. Extra safe. And useful. I keep the CHEs on top of the lids now that winter is on. Totally worth it just for that, truth be told.
8" - 10" walls would be very marginal for my girl, even with nothing in the cage. When she stretches she's like a foot-long hotdog. She's got a lot of reach. She can easily climb on top of her igloo or her tunnels. From there, it wouldn't take any effort at all to go over the side - and 44" to the floor. She could easily escape the 14" high cube walls, even with the 12" sides that I have set up.
Her lids are simply wire, linen-closet shelving that I got from Lowe's. Cheap, and easily cuts to length with nothing but a hack saw or cut-off wheel. I use the 16" deep shelving; one piece for the front and one for the back. Perfect depth for use with 14" square cube grids. I got lengths of shelving that were long enough to make both front and back pieces when for one 2 grid x 4 grid cage.
This is a pretty old pic. The walls are shorter than they are now, but it shows the tops well, and I have the CHEs in position.
http://serenebreezes.com/Sophies-pictur ... ned-up.jpg
Another picture, closer view of front and back lids, with CHE in frame. Except for cage cleaning, I only take off the front lids. That's plenty for full access to the cage. I just move the CHEs to the back. NOTE: Be careful about setting CHEs 'aside' on carpet / floor / table. They can get very hot and forgetting that could be seriously dangerous.
This is more of a general room tour, but it shows the wall heights and back lids pretty well. You don't see the CHEs in this video. They are placed right on top of the lids. This video was made in the summer, so the CHEs were stored away. Some of the other videos show the CHEs.