Due to the relatively low number of Special Inkwells needed to create this item, it is quite difficult to get a +15 bonus compared to other Rank 4 Fusion items that require 10 or more bonus-providing materials.
The other Hat of Innocence ingredients can provide bonuses of up to +4 each so a Topknot Hat+7 is the absolute minimum requirement to fuse a Hat of Innocence+15, however as both are rare drops it is easier to farm better Special Inkwells before fusing this item.