I made plenty of pack-n-play and crib/toddler bed size sheets, because they were actually cheaper than buying, depending on your fabric choice. You don't need wide fabric - even the crib size can be made with 45-inch wide fabric. However, I've never made sheets for twin beds, because I'd either have to find wide fabric (maybe more $$) or piece extra width on - either way, it'd be more $$ and no longer worth it to me. But if it's a special fabric that your GS would really love, then it shouldn't be difficult.