From the looks of it, it should be similar to the original polarguard, just heavier.
The bag is very thin. I would say 60dF is more likely the rating depending on how you sleep and why I have the thermal bivy in the pack.
I do have a #7 montbell synthetic bag and this is just a bit thinner.
With the thermal bivy this home made bag should be good down to 45-50dF depending but then you are up to 21 oz.
One thing for sure, it would need a sewn in footbox to go to 45-50.
I think I am going to build the real deal next, but probably a waterproof down quilt, if I can keep it at 12 oz. then do away with the thermal bivy or maybe get the lighter thermal bivy as an extender.