Say you are going to enchant a device in your mental construct laboratory. In this case the device is either for use within the mental lab, or it is something that you intend to use with your Gateway to Reality from a few posts up.
You don't need to bring in material from the physical world, in fact it would be disastrous to do so, you'd need to keep some sort of magic on your device all of the time to prevent it from shifting into the real world. That means both effort and warping.
The vis to enchant it would need to be real, but once expended I don't see any reason why an enchantment created by the physical vis would have any difficulty staying mental.
(side note:I think that a pesonal vis source of mentem vis that is harvested from within one's own mental constructs would be really cool.)
So where does one get a mental hunk of wood, pile of gold, or cunningly crafted ruby ring? You could rule that it's your imagination, you can imagine anything that you want for free at no cost. That's an option
We do have a precedent in the rules for a different option. You can create a mental object in your memory palace by studying a real item and making an art of memory roll. This roll can even be made without the art of memory skill (TMRE p25). I like the idea of the magus carefully using their concentration to study a physical object, and in so doing, craft a mental one. I like it better than just saying that you can imagine whatever you like.
But what if you character wants to put an item into their mental space that does not have a real world equivalent, or at least not one that the character can study? I'd have the character make the same art of memory roll with a DC 3 higher, because I think it would be harder if they don't have a physical object to work with.
I can think of a few ways for spells to help with this process. Rego Mentem base 4 can control a target's mental state, this might aid in focus while committing something to memory, with an appropriate duration I'd allow the character a +3 bonus to their art of memory roll.
Creo mentem base 4 can place a thought within a mind. While this thought would naturally have a duration, creating such a thought would mean that the character would never work with the higher DC's that come from not having an object to work from (you know, the penalties that I made up 2 paragraphs ago). Imaginem magic could function in a similar manner.
If you were really bad at memorizing things but had lots of vis (say, for instance, you had a personal vis source within your own mental constructs that was difficult to bring to bear on the physical world) you could create mental objects with a momentary creo ritual.
Rituals need to be level 20, at base 4, range personal, duration momentary, your most efficient level 20 ritual would create 1,000 mental objects I'd let them vary however the caster wanted. I'd also be pretty generous regarding what makes up a mental object.
Going deeper down this rabbit hole, If you had supplies of mental base materials altering them into other things using a mental version of rego craft magic would be reasonable.