Hi,
Similarly to AV8TOR I glue a 6mm plywood flange to front of firewall. This holds cowl very well in place. To mount the cowl I laminate two pieces (one for each side of fuse if its flat obvoiusly) of duraluminium L-shaped bar (1mm wall thickness & 80-100mm long). They go under the flange. Cowl in mounting area is reinforced from inside with two stripes of thinn 0,1mm brass laminated under fiberglass stripes. Put cowl in place and drill through reinforced places 4 holes for self tapping screws.
I use this method in all my 1/4 scale aerobats.