Making Nailer Blocks?

When fitting the spars, ive noticed many people have lovely shaped 'nailer blocks' (if thats the correct term) between each spar centre. I would like to fit these so that the internal skinning is nice and rigid. (And to give me more meat to attach outer skin/alu). Thing is I havent got a clue how you would cut these so nice to fill the gaps between the spars, whilst keeping the curve so good. (Inner curve especially)
Can someone explain how these nailers are created? My wood working skills arent too bad, but just never had a reason yet to make any!
Alternativley, Ive been looking at this guys build

I wonder if it is easier just to build the side walls with the perimeter framing first, fit the inner wall skin/insulation THEN fit the spars.
If I did this, do you just shape the outside of the framing to the curve of the outer skin by using a router bit with the bearing at the top next to the collet?
Anyone care to add?
Ive tried searching, but you can imagine typing in 'nailer' brings up 1001 nailgun threads lol
Really would appreciate any advice.
Can someone explain how these nailers are created? My wood working skills arent too bad, but just never had a reason yet to make any!
Alternativley, Ive been looking at this guys build
I wonder if it is easier just to build the side walls with the perimeter framing first, fit the inner wall skin/insulation THEN fit the spars.
If I did this, do you just shape the outside of the framing to the curve of the outer skin by using a router bit with the bearing at the top next to the collet?
Anyone care to add?
Ive tried searching, but you can imagine typing in 'nailer' brings up 1001 nailgun threads lol
Really would appreciate any advice.