tirewalls come standard as SObject in Bobs.. just make new textures for them.
for making a tirewall shape in sketchup.
start with a flat square, put the texture on it.
but on the white bottom side.
Never put textures on the blue side of a shape, than it's inside out.
group that square.
than divide the square in say 8 equal vertical line.
Sketchup will always find the absolute middle of a shape.
So start with the middle.. and keep dividing.
fold the parts 45 degrees each.. so you end up with a 8 sides cylinder.
close the top put the top and the top texture on it.
Now you have a perfectly aligned texture wrapped around it.
than export it to DAE or 3DS.
import that to the Xpacker.
take the standard tirewall from bobs.. and replace the object with your own.