Hi Modelnut,
the method I use to thin down slablike struts from a kit is to place a strip of double sided tape to the edge of a modelling mat or work surface.I then reduce the strong adhesive quality of the tape where the strut is placed by pressing on the tape a couple of times with my finger,this allows you to prise the strut off after working on it without distorting it.Then the strut is placed on the edge of the tape nearest you.Take a craft knife and gently scrape along the length of the strut,this is called 'adzing',an old wood working term I think.When one side is finished to your satisfaction,turn the strut over and do the other side.Finish off with a gentle sanding if needed.Takes me about 5 minutes per strut.
HTH.........Dave.