Snuggle time can last for quite a while, especially if your guinea pigs are real couch potatoes. It's not uncommon to see them really sack out on your lap for an hour or two.
Accidents, however, sometimes happen...especially if your furkids drank water, or chowed down on cucumbers or melon, prior to settling on your lap. While they try to be polite and send warning signals that they're ready to be put back in their cage, sometimes their little bladders just can't wait until we humans get a clue and read their signals. And then we get soaked (don't worry, it comes out easily with a little hand washing or regular laundry cycles).
Even the best towels or fleece sacks can't quite protect us. To stay dry, try putting a puppy training pad in between layers of a towel, or between your lap and the fleece sack, to effectively soak up any wetness. These pads also make a great liner for the bottom of travel carriers.
Another option is a cat litter box liner that uses baking soda for odor control. Arm & Hammer has a product in this category, as does Lambert Kay. Like the puppy training pads, these are quite handy at the bottom of travel carriers.
These pads and liners are generally on the big side, so you'll either need to do some artful folding to tuck things away from your pig's impulse to chew, or consider cutting them in half with scissors.