Do you sew at all? If you wanted to up the insulation of the FF’s kid bag, you could take some 2.5 oz/yd2 Apex and a single layer of UL fabric and sew that onto the bag. Downside would be more weight and bulk for not much of a insulation increase.
Upside would be that it would handle moisture better (provided Apex is on top) and it would be warmer by like 15 to 20* or so and no complected/difficult sewing and probably cheaper than having a higher down content bag custom made.
Wearing one’s puffy jacket (and/or pants) while sleeping also helps.
Regarding your current bag, you could also try combining it with a SOL Escape Bivy. I did a somewhat recent trip with a lows around 32* F and used a 50* bag inside the bivy while wearing a puffy jacket and I was fine. I also cowboy camped (no tarp or shelter above me). The only part of me that was cold, was my face due to wind. I should have brought a balaclava. (I do sleep warmer than the average. For reference, I keep our place at 65 to 67* F in the winter and sleep nekked with only a thin sheet over me and I’m almost always fine with that unless I have a cold or the like).
In your case in relation to the above bivy, you would probably want to put the bivy inside the bag if you were going to try that. Or at least, try it both ways.