Dreamscape where did you find that template or did you make it yourself? I am trying to make my own for the exact same reason... waaay too expensive to purchase a kit. ($550-$1100 plus shipping to canada)
I also referenced Wizardofflight on deviantart.com - partcialrly their "rifle 2". This shows the cross sections really nicely too.
I definitely recommend Ronseal multi-purpose white wood filler. But if it did this again, I would use that for all the filling, and not hot glue to block up the corrugation in the card. It's a pain to sand which means it makes getting a smooth surface against the wood filler more tricky as it's two different materials that sand at different rates. I sanded the wood filler with a 120 grit.
The plastic filler primer is very nice - and once sanded with 800 grit leaves an almost plastic smooth finish to the surface.
Now started on the barrel. Base layers of card done, and started to build up the wood filler. Havent quite decided how I'm going to do the bumps on the top, possibly foam strips or hot glue with filler over the top.
I won't attach this to the base until I'm happy that I've got the right shape.