No, you actually have to clamp it. Holding the panels isn't sufficient. Wood glue creates what is called a mechanical bond between the two pieces of wood (not a simple chemical bond like is normally thought). What happens is that when you glue two pieces of wood together, the wood glue seeps into the fibers of both pieces and fuses the wood together, thus making the joint actually stronger than the wood being bound together. The problem for us is that MDF usually has a thin "veneer" on the top where it's been sanded, so it doesn't seep very far on non endgrain pieces. However, the joint still does bond stronger than the wood itself; myself and many others have actually tried it and shown that it does work.
I've built a few boxes in the trunk of a car, will be doing one in my little brothers car very soon.... It's a very tedious and time consuming task that requires quite a lot of patience.... you'll need to do ALOT of pre-planning, measuring every aspect of the trunk making sure you don't miss anything that is either sticking up or down a little more than everything else and getting in the way.... then there's a tool like this one: http://www.lowes.com/lowes/lkn?action=productDetail&productId=255535-39450-R3&lpage=none
It greatly improves the ease in screwing the panels together.... worth EVERY penny!