There are a lot of great products that fit into a MTB steerer tube but these are usually aluminum and are almost always open at the bottom of the steerer. Gravel bike forks typically are closed at the bottom of the steerer and many have carbon steerers. The new Granite Design Stash RCX uses a compression plug in lieu of a starnut and is is designed for forks without opening at the bottom of the steerer tube.

The Stash RCX features:
-8-piece Multi-tool: 2, 2.5, 3, 4, 5, 6, T25, Flathead
-Spoke key: 0, 1, 2, 3 with valve core removing function
-System weight (incl. multi-tool): 110g
-Multi-Tool weight: 57g
-Color: Black and Orange
-Stash RCX fits forks with or without the open end at the bottom of steerer tube.
-The inner diameter of the fork steerer needs be round shape and measured between 23.5mm to 27mm.

In stock on August 27th for $54.99 at Granite Designs.