Yup! that looks like the ticket!!!
Your likely still going to lose heat thru the mattress, especially foam. It'll compress giving less insulation.
Like others mention, double up the mattress, use Reflectix or closed cell foam.
In my FP I lined the entire bed area with closed cell foam (the kind you get at Harbor Freight to walk on in your garage/shop), then I use a dreamtime mattress.
I was plenty warm up in Maine last month subzero temps. I do normally sleep with a vapor barrier & wool hat plus use a 2nd bag as a topper only as conditions warrant.