In the many posts on this subject- I turned the H18s on their sides put glass cloth and epoxy on the INSIDE thru the porthole. Very messy to get it flat and unfolded against the crossbar mountings, but will be a LONG time til it cracks again.

Dacarls:
A-class USA 196, USA 21, H18, H16
"Nothing that's any good works by itself. You got to make the damn thing work"- Thomas Edison