After talking to friends that also own a Senseo machine, I realized that not many people know about a fantastic accessory for the base model Senseo - the extra large water reservoir.  Simply replace the old tank with this much larger tank (no tools necessary) and easily double the water capacity of your machine.  Now you only have to fill the machine half as often.

I've owned the extra large reservoir for over a year now and it works wonderfully.  It fits the shape of the Senseo well and is very easy to handle and not awkward in any way.  They can be found inexpensively on eBay and shipping is usually low because of the low weight of the item.

The picture below shows the original reservoir on the left and the extra large on the right.

Even though it looks drastically larger, it fits the Senseo machine well and the grooves in the side make it easy to handle.