I once made a forend wrench out of scrap aluminum by turning a cylinder in the vicinity of 1-1/4" long and 1" diameter that that just slipped inside the nut/tube as a pilot/guide, then I made a flat rectangular piece roughly - and I am relying on memory - 3/16" thick x 1" wide x 3" long that just fit in the notches and stuck out the back far enough to clear everything and clamp a BIG monkey wrench onto. Then I cut a slot about 1/4" deep across one end of the cylinder, the slot being just wide enough for a press fit with that flat piece.
The flat piece was then lined up to be even on both sides of the cylinder and hammered into the slot. Yeah, it took me about an hour of screwing around on a tabletop lathe/mill... but it got the job done. IIRC, if I was a more careful critter, I could have dispensed with the cylindrical pilot altogether.
(After a short search...)
Oh, hell... I knew it was still around here somewhere. Here it is, and the measurements are not exact, but in the ballpark.
Attachment 72906