I've gone for 3.60mm plywood to cap the centre-section as (a) the sheet-metal shop is closed at weekends and (b) ply is three times cheaper.
This done, tee up your cantilevers as they'll appear on the prototype, so for example I've retained the best cut of ply for the topside, whilst this here will be the base.
As a result we've gone right foot forward as we'll be entering the air vehicle from the left side. This is standard practice in the air and at sea and stems from the days that boats were steered with a paddle down the right or 'steering board' side.
If you're left handed you may then prefer to reverse the arrangement and bear in mind you can do this at any time prior to fitting the flight-controller ~ and even afterward if you're prepared to operate with an inverted controller affixed the underside.
We need now to mark up a drill-hole to fix the ends of each spar to the adjacent one, and so there is an inch-and-a-half to come off the 15" perimeter length, besides 5mm for the width of the M6 threaded insert that caps the spar.
This provides for a 6mm drill-hole around 340mm from the end of each spar in order to secure.
Don't be afraid to sub wood here and there for prototyping. Germany's first fighter jets were constructed in wood, as was the Mosquito: both among WW2's fastest aircraft.
