I have the same problem running hose for the heater through the firewall. I see IC2 and some others seem to use bulkhead plumbing but I did not know where to get that sort of metal tubing interface. I am open to suggestions for a source of those nice looking fittings but I have already been through several hole enlargements and have been able to use three ever-larger hole saws. I could still install a metal bulkhead with tubing fittings to cover the holes I have now. A complication you did not mention is that the 'glass is much thicker that would be the case with a metal firewall and I have also been through two sets of rubber grommets which really do not work because the groove in the grommet is not wide enough to accomodate the firewall 'glass which is about 3/16" to 1/4" thick. I would also warn you that it is not easy to cut around existing wires and I was lucky to be able to run a larger hole saw through an existing smaller hole (twice). I have used a Dremel tool to clearance my 'glass body in several places and it will cut fine BUT you better have a steady hand because the twist of the cutting bit makes it more difficult to control the path of the bit than it would be to just draw a freehand line. If you choose this method I recommend you practice on some scrap fiberglass for several attempts until you think you can control the bit. Thanks for this thread, it gives me the idea that I can use one of those bulkhead tubing fittings for the heater hoses if I can find out where to get them. However in your case I think the best choice is to find or make the multipart bolt-together metal or plastic grommets, the 'glass is too thick for most rubber grommets. The idea of a wrap around metal sheath may work pretty good if you can hide the seem on the bottom and put the fastener on the inside of the firewall. My main message is to not be very optimistic that you can control a Dremel cutting tool in a way that will lead to a finished appearance. Good Luck and thanks for some ideas from others.

Don Shillady
Retired Scientist/teen rodder