I use Cashboard, and love it.  It's a web app with a great UI, so I can use it anywhere.  You set up projects and tasks, and assign billing rates.  Entering time takes only seconds.  Generating and sending invoices is a snap.   It tracks outstanding invoices until you get paid, even automatically sending periodic reminders to the client. 

It's not free, but it's almost free.   :Thmbsup:

http://www.getcashboard.com/

I would be glad to help.

Another idea, which might be better than sketchup, would be for you to put your experiment on SecondLife.  Secondlife is another immersable 3D environment, but with more emphasis on social interaction as compared to Sketchup's emphasis on architectural design.

With your experiment on Second Life, people could be anywhere in the world and participate in your study.  (Or you could run your study from any computer in the world)

They have a special deal for educational projects that you might qualify for.

If you want to go off-line for this, I'm setting up a temporary email address that you can use.  (I don't want to post my real address here.)
qwert-at-onepost.net