Layering the mattresses, even with a sleeping bag, is not halachically acceptable because you would still be sleeping on top of the same bed.

Your separate mattress has to be clearly separate from your husband's bed, with physical contact between you while sleeping 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, is it possible to find 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.

We appreciate that your husband is sensitive to your needs, and hope you can find an arrangement that is both halachically permissible and comfortable for both of you. Please write back to us if you have any further questions.