as promised - here are shots of my solution:
This is the "shelf" i bolted to inside of the left one, I also added more support inside the "box" with plastic braces.
The bolts are covered with hot glue.
Here is the left lower part, with the velcro attached:
and the upper part has the velcro in the same place as the original prop:
The two together:
and here is why I made the shelf: (you can see the bolts here) -- the lower part will slide in between the shelf and the box, the friction between the two holds it in pretty tight.
I used the same technique for the right one, however, the two parts are attached together solid, they only split on the inside seam. They are bolted together with bolts that were sunk into the flamethower (jbweld) and through a piece of plastic inside for support.
You can see I also used bolts to attach the nozzle on the top - you should always use some sort of mechanical connection when attaching things like this, they are less prone to snap off.
Here is the velcro attached to the outside of the right lower part:
and in the inside:
and here they are both closed:
I still need to add my electronics, finish painting and the attachment of the fire parts...
I based this type of attachment on the pictures of the actual props. The velcro is industrial type - so it should stay "sticky" for a while, but I can always replace it if needed.
if you have any questions - shoot them on over!