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.
Small update - I've now covered most of the surface in woodfiller, and sanded it back. I've added some detailed panels too. Really happy with the curved surfaces. Now weighs over half a kilogram.
Added the first extra piece to the base unit - and covered it in filler. Looks a mess but should come up nicely. I really should add the sides to the rifle's stock.
Sanded the main rough bits out. Ran out of plastic filler primer so resorted to some standard grey car primer to highlight scratches and nicks. That ran out too.
Still a lot of edges need sharpened up, and lumps/bumps to be filled/sanded.
But I'm really happy with the results so far. Unsurprisingly it weighs 650g and feels like a piece of wood. Guess that's to be expected as its half solid wood filler!
Haven't done a great deal recently, but now built up the top of stock and added some more detail. Ill be starting on the barrel after I've sanded down and primed the top of stock.
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.