Rob is right, if you mess - taped diapers are a must.
Sure, duct tape can be used with effect to tighten it up the waist. But do not forget that you have to leave some "room" in the diaper when putting it on.
The diaper should fit snug around legs and waist(hip is too low). Put it on in way it is not pressing tight around the butt and croth - leave some space there. Regardless of consistency a mess cannot be absorbed, it has to move somewhere. Having the diaper too close at the body can lead to the mess leaking out the legs or back no matter how much duct-tape you use. A protective plastic pants might also be a good extra protection just in case of a leak. Matress protection should also be mandatory.
Cheers