That rectilinear form is probably best obtained with TiM's method .  Fill it with model clay then use that as a form to build it up with clear/smoked sheet stock .

   This ain't novice class either way but doing it is 80% the fun IMO 

 ....If it were me and the kit is not that great and I was gonna start scratch-freaking on the canopy I would just keep on and trick the thing out .    More enjoyable for me when I'm not worrying about matching original .

    Post some picks !   Especially pics of methods and madness .

Good luck !

Keith