Hi jackback,
The only thing I know that would “feel” like a cotton futon would be another cotton futon.
I don’t know the details of your poly “cloud” topper so without more information it would be very difficult to approximate it. Do you have any more information about it (thickness, foam density, ILD/IFD etc)?
When you are trying to duplicate another material it will usually involve some trial and error because “feel” is so subjective and individual. From the sounds of it the latex topper is softer than your old topper or it could be a different thickness as well. If you are using the softest latex and you are more “in” the topper than you prefer then you may need firmer latex or a thinner topper.
If you are using a latex base layer then it will also “feel” different from a cotton futon which has very little give at all … especially when it has compressed.
Trying to “duplicate” a mattress or sleeping system using different materials is a serious challenge and would take a lot of trial and error based on your own subjective perceptions.
If I was in your shoes I would look for a mattress locally that had the feel and performance you were looking for in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) and once you know the specifics of what is in it then you could either purchase that exact mattress or you would be in a better position to approximate it online using the materials and design that was as close as possible.
There are a few sources of futon mattresses in post #2 here and some of these may carry cotton futons and then all you would have to do is figure out how to approximate the topper.
I’m not sure where you are in Northern Michigan but post #3 here has some of the better options and possibilities I’m aware of in the NW Michigan / Traverse City area.
Phoenix