How to mount stock to 5 axis plate?

I purchased an Hy-3040 a while back with a 5th axis.
Never had to use the 5-axis, but would like to play around with it, problem is, I have no idea how I would mount stock to it.

If you look at the picture I uploaded, its a plate, but to access the screw holes, the plate needs to be off, but if stock is mounted in middle, you cant fasten the middle bolts back to the assembly.
I've tried to find a chuck for it, but it would need a custom adapter to mount one.

They have this video on youtube,
https://www.youtube.com/watch?v=DwoOYhp2txw
but they don't show how it was mounted or unmounted.
They have a similar one with foam, but I suspected they used glue.
Any ideas how they did this with aluminum?
.
They also have this video
https://www.youtube.com/watch?v=dQLt-UOuw_w
which is the only way I can see how to mount, but its not like you can purchase aluminum stock like that.

Only thing I can think of is to create an adapter with a dove tail or something.