Here's what I use for the larger slabs:
http://www.the2buds.com/susllgpc.htm
The slab fits just fine with not too much extra room. The drawback is that unlike typical sleeves for graded cards, these can't be sealed. But even with the open end, I find these work well, especially when they're stored in a box. Basically, I put up with not being able to close the sleeve because it protects the slab.