Flashing adapter help

[Fire6000]

I am switching from a heat n glo zero clearance fireplace, and will be installing a accentera 52i in its place. The zc fireplace and the flue pipe are out so I am starting Basically from scratch.
My flue is venting up out a false chimney, made of wood and stucco. Existing was using sl300 pipe, so my flashing has a collar around 11.5 inches round. I am attempting to not have a new flashing made if possible (because I am cheap). Chimney is probably about 3 foot square. Seems like having dome sort of adapter made would be cheaper than new flashing.
Does anyone have ideas how to adapt my flashing to fit the 4" pellet vent and be rain tight? I have a piece of sl300 and storm collar to use if needed.

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[DAKSY]

Get yourself a couple of different sized storm collars - OR one with a small ID for the pellet vent & large enough OD to cover the collar on the chase top. Either way, apply 30 yr silicone LIBERALLY to seal everything.

 

[Overfireinthehole]

Various storm collars and np1, or a top plate cut round with edges bent over sides of your 12 inch hole and lots of np1. The solution is always lots of np1 or Vulkem.