how to make a divider for a timbuk2 bag?

How can i make a divider for my dividerless timbuk2 messenger bag? 

I'd use the divider for a laptop, or camelback... if details help.
o and a large!

Could i do it with velcro? 


Sure, velcro would be fine. Just make a divider panel with flaps to attach the velcro to, and you're good to go. The hard part would be attaching the velcro inside the bag. The sewing could get pretty cramped.
One thing that might be easier to assemble and more secure would be to use heavy-duty snaps instead of the velcro. The snaps and the attachment tool can be had very inexpensively at any sewing/fabric store, usually as a kit for just a few bucks.