Top Bag? Need more insulation

A top bag would definitely work. I used to use a MH Phantom 45 with a nunatak arc AT (40* half bag) and patigonia nanopuff inside. The coldest I got down to was 24* and it was just pushing the limits of warmth for me but I also had all my clothes on. You would want the lighter bag on top to avoid compressing the insulation of the bag underneath. There's tons of different options to use as a top bag. Thermarest makes a coforter like you mention but it's heavy. A simple down quilt will probably be lightest.

You could get a slightly warmer pad and it may help some. You're pad supposedly is good to 35* so it would help but it would be expensive also. It looks like your bag has no hood either. Do you wear anything on your head?

As far as merino bottoms mine weigh 9.1oz and they're pretty small. Down pants will be lighter. Down booties may also help if your feet get cold.

Really though to achieve the actual rating of pretty much any sleep system I'm aware of you need to wear additional clothing. Wearing clothing you're already carrying is going to be your best bang for the buck and lightest option. Sleeping with minimal clothes is comfortable though but I find being warm is more comfortable.

Forgot, I also have an MB EXL jacket that I carry and use for insulation. And I do have a Costco down jacket (9 oz) but that still leaves my lower body with no additional insulation. Maybe I could wear the jacket on my legs?

I took my WM highlite (35F, 16oz) camping last weekend to test it to the extreme. It got to 19F, i had thick merino socks, Patagonia cap. Bottoms, merino top and a synthetic jacket, MH dome beanie. I had a neoair and cheap ccm mat. I found that my feet and legs were fine, it was more my hip and torso was cold. When I couldn't take any more I placed a wool blanket over my top half and was fine. I think some kind of half sized synth or down blanket would work for you. Or a vapor barrier eg. WM etc.

Went in my bedroom and tried on my MB EXL and my Costco Down Jacket on my lower half. I realize they are jackets, not pants. But just thinking here. Anyway, if you put the zipper to the front, the arms are a little short on the legs, but they do work fine and zipping up the zipper you can pull the jacket up to your waist and get some significant extra insulation. So maybe that's all I really need.

I do like the idea of a half (or even three quarters) down blanket. That's pretty cool.

I’ve got a down blanket(and a bivy) on the way from Montbell Japan in a medium.
(There is a small but no large yet)

100x146cm
20D Water Resistant Nylon Taffeta
196g
650 fill down:( (I can’t find the fill weight)
Currently available in any color as long as it’s green.

And here is a link to the Montbell Japan website.
It’s in Japanese but has a few photos.
(I love the sort by weight function)

That's a pretty small blanket 39.3" x 57.48"

Ways that I extend warmth when temps drop:
1) Something very warm for the head, preferably covering most of the face
2) A warm sleeping pad. Having a warm top layer doesn't help if a lot of the heat is leaving through the ground.
3) A puffy vest adds a lot of warmth. A sleeved jacket adds a little more but not as effective for the weight. Your body is far more sensitive to cold in the head torso area. Cold arms and legs are less of an issue.
4) Underwear layers and double socks may seem uncomfortable at first, but you adjust after a while and you very much appreciate the clothing layers when you have to take a pee at 3:00AM.

A few thoughts:

*The ClearView pad may be part of the problem in staying warm. Try an insulated air pad or something with a higher R-value. You could add a thin foam pad to the top, but that will get into losing returns as with the next issue.

*Adding insulation will be warmer, but you will get losing returns for weight and performance. Somewhere along the line you need to change out that bag for something with lighter weight and more loft. Unfortunately, high performance insulation in clothing and sleeping gear is one of the more expensive parts of an UL kit.

*Wearing clothing to supplement your sleeping bag is Ultralight Canon Law. You don't *have* to, but it does squeeze the maximum performance for the weight via multiple use. Many will sleep in *all* their clothing to get the most from their kit for the weight and include it as part of the calculation for sleeping temps. I like Power Stretch long johns and tops for good warmth and comfortable sleeping. They are fantastic under rain pants for cold morning camp chores and meals, but too warm for hiking unless it is very cold.

I use a blue torso-length pad under the ClearView. Probably be better on top but then I don't get the advantage of using it to protect my lightweight pad.

I'd really just like to find a whole new solution. But with using it just a few times a year, it doesn't pencil.

I gotta go look at that green blankey.

bring both poofays … use one under/over (over is better IMO for condensation purposes) yr quilt for yr legs … wear the other for the the upper body …

bring a cheap foamy pad for additional ground insulation …

carry a nalgene just in case and the fuel to boil water should you be worried …

should you decide to make a 40F quilt … itll add 15-20F to the bag …