When I did my sons bulkhead I cut a piece of aluminium about 20mm wide the shape of the top of the firewall marked and fitted the rivnuts into the strip I had cut. I then silicones it to the inside of the firewall so that when I fitted the bulkhead there wasn't a gap on the engine side of the firewall.
There is definitely a knack. When I finally got mine out I put two layers of aluminium Tate around the edge of the honeycomb to try and help soften the edge.
If you use the gunson don't put fluid in the reservoir but keep the master cylinder topped up then if you get a leak on the gunson side you won't spray brake fluid over anything. I now use a vacuum hand pump to bleed the system on my car and also on my sons car.