I modified your pic to show how I would install a hatch. This way you can install tri stock on the top edge of the firewall and have some top side support. When using a sport engine firewalls come out because they were not installed properly, not because of a hatch.
I blacked out the big hunk of ply in front of the firewall because I couldn't figure out why it was there. If it is for the CG I would find another way. It looks like your plane is going to be heavy.