What is best way to secure an armor diorama (10" x 11") to the bottom of the shipping box?? Blue Tack??
Thanks in advance

I don't think that blue tack will be able to hold it in place.

How is the diorama constructed?

If the bottom is solid enough to accept screws I would attach it to a slightly larger board or some pieces of wood and then secure those to the bottom of the box.

If the diorama has solid edges, like a nice frame, I would use sheets of styrofoam along the edges, pressed down from above.
Use strips of styrofoam to center the diorama on the bottom of the box. Sheets of styrofoam cut to size to match the four sides of the box. The sheets need to rech in over the frame of the diorama. Fill with styrofoam between the sheets and the sides of the box if necessary. Secure the styrofoam sheets to the sides of the box or use some inside framing to press the sheets outwards.

Cross section sketch of the general idea


Cut the long sides to match the inside of the box and cut the short sides to fit inside the long sides, similar to the framing piece in the middle.
The design can be developed by cutting tabs and slots to make the whole thing self-locking. Depends on how much time you want to spend on it and if it will be re-used later.

Don't know if this is the best way but this is the way I would do it if the diorama base can't be secured with screws from below

/ Robin