A book bag? This is what I use to weigh down the front end. She complains a little at speeds >100mph but it all works out in the end...
On a more serious note, because of the mid-engine balance, a Boxster generally should feel very well planted at triple digit speeds. What is the status of your suspension? The lightness or vagueness that you're feeling might be from worn shocks and bushings. Re-freshing the suspension might improve both high speed straight-line and cornering feel and performance.