14~18oz. (400 to 500g) certainly seems doable but you'll have to be careful when choosing the power system - most likely the LiPo will account for approx. 100g. So I am guessing you'll end up nearer to 500g. The Hacker is a good choice.
I recently built a smaller monoplane with similar looks, the Owl-RT which you can see in my blog here at RCGroups, using 3mm Depron. With a 31.7" WS it ended up just shy of 10oz.
I noticed you are being very careful to carve out your crutch parts; when building the Owl-RT I found out that this is not really required, as Depron foam is extremely lightweight, so weight saving are minimal and the parts loose some strength. It does look good though.
There is a thread here in this forum with the title "Depron tips and tricks" or something like that, you may want to take a look at it.
Good luck with the building and keep us posted, I'll certainly be following this thread!