Javan, is your use of the higher temp wool under a cast liner a requirement for a welding forge, or a result of your welding sessions being many times longer than most peoples? The reason I ask being that I can buy regular inswool and Mizzou locally, but I'm having a hard time finding the higher temp version short of ordering from the US at many times the cost.
In my experience, the 2300 degree material simply doesn't hold up long term for long welding sessions. It'll work for a while, but wears out an order of magnitude faster, and really isn't forgiving for temps above the carbon damascus requirements.
I can also tell you, that after switching to thermocouples, after years of welding by eye, that it's brutally obvious that most welding forges people are running without them, are running at 2400+ degrees typically. That high yellow/white color you see inside a forge, is well above 2300, and the lower temp stuff is not meant to be run continuously even at 2300, it's continuous rate limit, I believe is 2100 degrees.
The 2700-3000 degree versions, will last 10x as long and don't break down and look like cotton candy if your cast-able gets a chunk blown off.
As far as a cast or troweled liner of castable, that's mostly for heath, and durability. Although a good insulating castable gives you some extra insulative value also, as opposed to being a further heat-sink.
Swinging around really long billets, gets destructive, and the heavier they get (and the longer you forge), the more likely, in my experience, you are to stab the end of a billet into some wall or corner of your forge. I actually line the floor of mine (I have to use a horizontal forge that can heat up the entirety of my billets, for using the rolling mill), with split extreme duty high alumina hard firebrick, and the entrance with hard brick also, because otherwise they just get destroyed, not from flux but from the billets themselves.
If I was building another vertical welding forge (which I believe is the best bet for most makers, without a rolling mill, and what I used for years also), I'd want the high temp insulation, Mizzuo liner, and a entry and exit made of insulation backed high alumina hard brick, with steel reinforcements, and a bottom made of the same, preferably hinged tops and bottoms for access.
If you can only find the lower temp ceramic fiber, I'd want to cast the liner, instead of troweling it on, and I'd want it a minimum of half an inch thick.
Regardless, I'd highly recommend 3" of insulation to anybody, over 2, but if you just do short sessions (2 hours at a time or so), it may be overkill. Guys I know that run large pottery kilns with 2,000,000 BTU/h in burners, running for a couple of days, utilize like 6-8" of insulating brick, and often cover all that with another 2" or more of ceramic fiber, and seem to always wish they had more. Even with 3" of high temp blanket, and a liner around my forge body, after 8 hours, the shell will still be hot enough to sear steaks, cause immediate and severe burns, etc.
They've got ceramic fiber rated at 3100-3200 deg now with continuous use limits at 2800 now, I plan to try and source some of this for my next build personally.
Also, I will add, that when sourcing this stuff, if you can find a semi-local industrial refractory supplier for the manufacturer, you can buy any of these things for usually half of retail, and avoid the most expensive aspect, which is typically shipping. Usually whenever one of my friends is needing this stuff, one of us will go to Charlotte or Atlanta, and pickup a box or two for all of us. Check with the manufacturers of each for their distributors, which often are different.
Bear in mind also, that there are a few different manufacturers of the same basic materials. Checkout Saffil, it's 96% Polycrystaline Alumina Oxide, with a 2900 degree continuous use rating, and 3600 degree melting temp, this is a product from Unifrax.
There's also some 3000 degree rated "Kaowool" branded products, like Kaowool Gold, but I'm not sure which are available in blanket vs bulk form.
