I would consider Tiers of this, much like Companion and such. This could either raise the WL the item is allowed to be or you could do properties and each tier gets to choose one from a list of properties.
As a presumably important item to the character, it could also increase in power (WL) with you, this would also make the feat worthwile at higher levels.
A quick example:
Cost: 2 points
Tier 1-3: None
Description: You possess an item of noticable significance. This could be a heirloom of your familiy, a gift from a benefactor or you found it lying under a single beam of sunlight, seemingly waiting for you.
Effect: You get an Extraordinary Item. It’s WL can be equal to half your level + the Tier of this feat. If it has other Items as a base (like a weapon or a piece of armour), you must treat these like normal expenditures.
The Item cannot have the Augmenting, Consumable or Expendable properties.
If the item has Banes, Boons or Attributes associated with it, they cannot be greater than the normal maximum attribute score for your level.
Special: If you ever lose your item, voluntarily or involuntarily, you regain the feat points that you have spent on this feat and may spend them as usual.
In addition to purchasing multiple tiers of this feat, you may take this feat multiple times and select a new item each time.
The Special Section is just copy-pasted from the Companion feat. Note that at 6 feat points and level 1 this feat allows access to a WL4 item, and continually allows for an item one or two WL above the norm for each level. I made it so that it keeps being important, but this version may be too strong, even with the Attribute/Bane/Boon level restriction.
After some consideration, I don’t think this would be too powerful. With the GM’s consent, Attributes/Banes/Boons one higher than the normal maximum could be allowed (note that this devalues Boon Access a little bit at level 1). The item obtained by this feat could almost always be obtained by doing a major expenditure by reducing your WL by one or by having the Wealthy feat (both work at the levels where one isn’t enough), the only difference would be the (potentionally) faster upgrade, accessibility and lower resource strain.
I’m using the recommended WL increases from the end of Chapter 1 for these assumptions.
I forgot to mention, but upgrading the item would probably need close to no time (like a level up) or a comparatively short ritual or crafting time, something like an hour per WL.
I disallowed the Augmenting, Consumable and Expendable properties in this because it may be a little weird to spend the feat points and upon use immediatly regain them, possibly to make a different Augmenting, Consumable or Expendable Item.
A workaround could be that an Item with any of these properties is instead usable once per day. This could be a self-refilling bottle with a healing potion inside it for example.
The second part of the Special section might need to go. More than one of these is likely to result in balancing issues.
EDIT 4 (i know):
Cost would need to be 3 in order to not flat out beat Boon Access for Boons of low PLs. In that case I’d seriously consider raising the maximum allowed Attributes/Banes/Boons level by one though.