Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

So what is the big hole in the front for - it seems the cord some out from the back - I assume my #10 did too.  Then I guess there is some kind of insulation inside or some kind of fabric?

The big hole in front is basically just for looks. Your cord comes out the back through the hole in the smoke box. Your engine should have a Belden style cloth covered cord.
If you watch eBay for a cheap model 35, you could get a nice firebox to use. The only thing different is you would have to use nuts with the small screws that hold the boiler straps. Boiler straps on the 35 are riveted and the holes are to big to tap for the screws.
I’ve taken some pictures for you to show how the firebox is assembled with the tray and the original asbestos insulation. New ceramic insulation can be obtained from Jensen.