Building a (bunk) double bed

Hi everyone! :laugh:

I am new to campervaning and have just purchased a converted ford transit.

As you can see from the photo, it has two fixed single beds, with storage space underneath.

I am wanting to make use of the high roof space, and build a double bed above the two singles.

I think it will need to be attached to the walls of the van, and possibly with some support (poles) underneath.

Am unsure how to go about getting this done, to ensure it is safe and secure.

Any thoughts/tips would be very much appreciated!

Thanks
Sarah