No trick, very primitive way, please don't laugh.
I cut the both side of the cowl to bend upper portion
to get clearence to the cylinders when putting the
cowl on.
You can see how I do it.
I think I could not do the same way for a fiberglass
cowl since it is not soft like thin ABS stock cowling.
I would cut the glass one in half horizontally from
the thrust line like a real airplane's cowl and fasten
both upper and lower halves using some hinges and
fasteners.
It will be more practical for maintainance and better
for scale look.

Tsutomu Mabuchi