A separate mattress is a good halachic option. It is clearly separate from his bed and physical contact between you while sleeping is impossible. But the mattress does not need to be on the floor! Is there room for you to purchase a second bed, or a futon? When you are niddah, he could sleep on his double bed while you sleep on a single bed. Alternatively, you could buy some sort of platform for the mattress, which could be stored under the main bed. If you find it comfortable, an inflatable air bed, which could easily be deflated and stored when you are tehorah, would also work.
Layering the mattresses, even with a sleeping bag, is not halachically acceptable because you would still be sleeping on top of the same bed.