Just had to figure out how to constrain objects to things (characters mainly) because, well, you forget if you don't do it for a while. Last time I constrained something was somewhere around this time last year. So I'm writing it down here in hope that I won't forget again (or at least if I do, I can come back here!). I even changed the colour of the important bits!
I was trying to attach one end of this box to Malcolm's hand because he's going to carry it. I will have to animate the constrain to unattached it when he comes to letting go of it. But we'll cover that another time.
Firstly I was trying to constrain it by Constrain > Object to Cluster but it wasn't letting me pick any cluster I created or any geometry to create a cluster to attach it to. I enquired the legendary facebook group TagerTalk for advice as I remember it was a fiddly thing to do so perhaps I was forgetting something.
To constrain something this way, first you should create a cluster on the geometry in the area you constrain to (pick point(s) Edit > Create Cluster or Ctrl + L), then create a null and Constrain > Object to Cluster. If the geometry cannot be selected (as was in my case), create a small point cloud (2 x 2 or something, depending on how big a cluster you need) parent it to a bone or joint or whatever is appropriate to get the desired movement and then select points and create a cluster.
Then select the object to be constrained and parent it to the null.
I did this and realised it was not what I needed. I needed the box to be able to rotate with the hand and using this method does not take orientation into consideration. Only tranforms!
So I actually needed to Pose constrain. And this is how I did that with the help of my tutor: As Malcolm does not have any selectable bones in his hand, I had to use his main wrist control. Select the object to be constrained. Click Constrain Compensation (CnsComp button under the constrain menu on right). Constrain > Pose. Done!
Malcolm character courtesy of AnimSchool.com