Fan duct for ZoneStar P802Q with J-Head hotend

I have a Zonestar P802Q, fitted with chinese J-Head hotend. I had problems printing PLA, after a few hours it jammed always.. No problems with ABS, though! But I figured PLA problem must be because of inadequate cooling, because the whole heat sink part was filled with melted PLA.

I found here one air duct made by widddo, but it was for the stock hotend. J-Head has slightly larger heat sink: 25mm diameter and 50mm length. Especially the length is a problem, the 40mm fan is mounted flush with