Varia RCT715 mount

I just purchased a Varia RCT715.  I need to be able to mount it to an L Bracket which hangs down from the back of a cargo carrier instead of from the seat post.    Is there any other mounting option other than the slanted seat post which comes in the box?

If not what size screws are used to hold the seat mount assembly together.  If I can match these, I make an L Bracket which I can attach with screws.   

The screws seem to have a 1.5 mm OD and are a machine screw of some type.  I haven't been able to identify.

Thanks!