Yes it is, we actually made one at the ATC last July at Wheaton Labs, it took more time fishing the fiber blanket out of storage than to make it. The interior isn't as smooth as firebrick, there are little ripples as you are fitting flat material into a circle, but it looks pretty good to me and was producing a clean burn. We were testing risers for upgrading the water heater for the showers. I didn't see any deformations or charring after use, and I'm assuming it's still sitting up there for use.
Cutting the blanket at 26" allows you to fit it in tight and tuck the cut edge together in an 8" pipe for a 6" system, and if a touch of trimming is needed to fit you can always remove a lot easier than add. My system will be 8" (dry fitted in the back yard for now) because my hand/arm is too big to fit a 6" system properly. Using large cutting shears/scissors is the way to go for me, over a utility knife. I'm now trying to find a reasonably priced source for insulated firebrick, I had a bunch of dense firebricks and halves, but want to use insulated ones for the sides and top of the burn tunnel and bridge.
Edit: the interface where the burn tunnel bricks and the square opening meet the round riser is a point where the metal pipe could be exposed to high heat, I'd make sure that you use bricks cut and fitted so that no metal is exposed to direct heat. Some will cob a few bricks around the riser to hold it in place with friction, so perhaps a layer of fiber blanket cut to shape that the riser sits on would help too?
Edit2: users at Donkey's forums https://donkey32.proboards.com/
have been using the 5 minute riser for several years now, all reports I've seen is they are holding up great.