[luatex] how to assign a width to a node of type "glue"

[Mico Loretan]

I'm trying to create a glue node that has a specific width. Unfortunately, I seem to be missing something (probably entirely trivial to somebody who knows how it's done).

Here's what I've tried so far:

local glue        = node.id ( "glue" )       -- 10
local gluespec    = node.id ( "glue_spec" )  -- 47
local gluenode    = node.new ( glue )        -- create the node
gluenode.subtype  = 0                        -- "skip"
-- what next??

I would like to make this node have a width of 0.03em (no stretchability or shrinkability needed).

The LuaTeX reference manual (p. 178) says that I should do something with the node's field called "gluenode.spec". If I understand the manual correctly, this field should be a "pointer to an item of type 'glue_spec'." However, there's no indication on how this should be done, and the explanation of glue_spec on pp. 173f. is even terser.

This is where I'm stuck for now. All help and suggestions will be much appreciated. Best, Mico