While physical gear (beyond said cardboard box) would not have an effect on how well you can be seen on the astral, there is potential for metamagics to do something of the sort. As for how you hide on the astral? That's a grey area of sorts, but I'd go with Sneaking+Logic [Astral] vs. Assensing+Intuition [Astral], with the obvious caveat that you'd need to be aware of the astral plane (perceiving, projecting, or dual-natured) to attempt it. The reason for Sneaking+Logic is that Sneaking is an Agility skill, and Logic replaces Agility on the Astral, so fewer leaps to go through there. As it stands, Masking can display a false aura (turning a mage into a mundane, or making a low-level initiate look like a dragon), but it doesn't keep you from being seen (observe in detail is different from 'spot', after all).
As for how the metamagic would work, I'd do it something like this:
Perception Filter
Prerequisite: Masking metamagic
The initiate is able to more effectively hide himself from astral observers. When using this metamagic, those using astral perception must make an opposed Assensing+Intuition [Astral] roll vs. the initiate's Sneaking+Logic [Astral] roll. The initiate adds his initiate grade to his dice pool for this test. If the creature using astral perception does not gain at least one net hit on the opposed test, he fails to notice the initiate's aura, though on a tie he may have a sensation of feeling that there was something in his 'peripheral vision' that is gone when he turns to look at it. If the perceiving creature spots the initiate, he does not have to make the opposed test again unless the initiate takes a complex action to 'hide' himself again. Using skills from the Conjuring, Sorcery, or Enchantment skill groups, or engaging in Astral Combat gives perceiving creatures a new chance to spot you, with a +2 dice pool bonus to their roll. Using this metamagic has no affect on perception tests in the material plane.
The basic idea of the metamagic is kindof like the 'perception filter' idea in Doctor Who. Your aura isn't gone or disguised in any way, but people just can't quite notice it as easily, like it was a half-step to the left of where you actually are, so to speak. An in universe analogy would be the astral construct mentioned in Feral Cities (page 20) that surrounds Elemental Hall, the local headquarters of the ASPS.