Hicksy my man thanks for dropping in mucka iv just done a qiuck demo box for you mate i hope you can see how its gunna be done wen you see these pics Click to view attachmentright just think this is the hole iv dug in my garage
Then the pre built box that will be the room is placed into the hole Click to view attachmentlike so ps iv left the lid off so you can see it then once the box is in place there will be a void of about 4in/5in all round the box and under it as it will have a brick under each corner il place sheets of steel mesh all around the box and over the top of it.( this mesh is about the same thickness of a cig ) the top sheet will be fixed to the4side sheets using stainless steel tie wire so it will look something like this Click to view attachment then it will be mass filled with concrete till all the voids and top sheet of mesh are completely covered this will be then the same level as the concrete thats around the top of the hole .so wen iv finished all that will be seen is a garage with a concrete floor till the next day wen il take a stilsaw and cut a 2ftx2ft hole though the floor and mesh and into the box i could drop inside the box and completely rip the wooden box out and id still be left with a perfectly soild concrete bunker hope you can see how it'll work mucka