erasing unused space from large drive



I currently have a large 100+ GB SATA drive as my boot drive. It is mostly free space and trying to run eraser on the unused space is a pita since it takes many days to complete.

So I was thinking. If I create a bunch of multi- GB file containers via TrueCrypt (using them only to fill unused space) That should reduce the time required by eraser dramatically. And since the file containers themselves are encrypted (and I don't care about the contents). If I find I need the space I could just delete one to free space up.

Does anyone have an alternative? Or does this sound reasonable?