Hi sbulla,
If you look at post #4 here under mattress covers you will find several sources.
There is a wide variety of different types including unquilted covers, wool quilted covers, and some that have a polyfoam quilting similar to yours (but thinner so foam softening wouldn’t affect the mattress as much). I would look at your options and talk with the sources if necessary to help you decide.
It’s too bad that you don’t know the ILD of the latex you have because it could help in deciding what type of topper you may need to add on top. If you did decide to go in this direction I would probably choose between a wool quilted cover (wool is very breathable and temperature regulating) or a stretch knit cover (which would stretch with the latex more and have less effect on its compression) and then you could decide on what type of topper to add based on how much more softness/thickness you thought you needed. Even a very cheap polyfoam topper from a big box store (which would be similar to the quilting foam you have) would be very inexpensive and while they don’t last that long they are very inexpensive to replace if they soften. Of course you could always add a higher quality soft topper on top such as softer latex or shredded latex which would last much longer. If your mattress is queen size then the seven comforts topper may be a good choice. I would sleep on the mattress first without a topper though to get a better sense of how much more softness you thought you needed before ordering the topper. There are sources of soft latex toppers in the same post I linked.
Edit: see post #52 here first if you are considering ordering this topper.
What you would need to do is remove the cover you currently have (you could use a seam ripper) around the polyfoam and remove it and the upper section where the latex fits and then take the bare polyfoam and latex layer and put them in the new 12" cover and zip it shut. You would then have a fairly firm latex hybrid mattress (based on your description of the latex layer) ready to add a topper on top to fine tune it to your needs and preferences.
Phoenix