thing is, there isn't much to put the dowels through even going through the side.
toothpicks through the side into the rear 1/8" lite ply and then some through the face 1/8" lite ply at an angle into the runner behind are about all you can do..............that and peel the covering back about 1/2" from the front and then use glass cloth to make it strong is best..............i'm using a YS fs110-S on my second SS now and it's been holding up.......
when i inspected the my first SS that crashed due to a Rx blackout, it was scary to see how flimsy the firewalls are attached............the YS surely would have parted ways by now had i not reinforced it.
but on the other end of the spectrum, i believe the flimsy firewalls on this airplane help to prevent engine damage in the event of a crash