hey, i understand about being scrawny! lol.  
if your trying to get linen, definitely make sure it says linen.  other wise it will be cotton.  course you could use that as a backing as well.  since its just to keep it from raising splinters, not draw weight, it would be cheaper to use cotton canvas instead of linen, though, not as traditonal.. i think
i have heavy canvas, dont remember the weight, that i used as a backing.  works pretty good.