Don't take the risk with blue tack, the fish tank will crack when the rockscape collapse.
There is a reefer call 'NAx0' in SRC and he is using light diffuser and cable ties to hold the rocks together. Instead of using a single large piece of light diffuser, u can use many smaller size ones and create an interesting rockscape. I used this method some time back when I setup my tank.
This pix is from his thread.