A shadow box is a great idea and they come with pins to help you keep light things in place,but what do we do about the heavy things?
Visions of epoxy and other adhesives might be running through your head but these can permanently ruin your pieces if you ever wanted to display them differently or destroy their value do the road.
So here is a solution I have used for a long time to display things of importance.
I like using this method but one setback is if you are really close you can see the string but most of the time you are not that close and I don't seem to mind
Step 1: Gather Your Supplies
Shadow box frame
piece of cardboard big enough to fit in the frame
paper glue, and embellishments to decorate your background
magic stretch string, the clear works best but the frosted works in a pinch especially on white things like my tiles
Step 2: Deconstruct the Frame
Step 3: Trace and Cut Your Cardboard
Step 4: Decorate Your Background
I have used plain cork on a lot of mine because of its simplicity and because it has no dye that could transfer over time
Step 5: Place and Mark
Now decide the best place to put your string to hold the items on.
For most pieces to will need a top and a bottom line.
So find a place where the line will be the most inconspicuous and hold on well.
mark a dot where the string will enter and exit
Step 6: Make Holes and Thread the String
string the stretch string through a set of two holes and tie in a square knot in the back.
how tight you tie them depends on how big your piece is.
Sometimes it helps to slide the piece in and pull against it as you tie.
Step 7: Place the Object Under the Strings
If they are too loose you can tie the top and bottom ends together to make them tighter.
Step 8: Put the Frame Together
sometimes if I cut the board a little small to cling to the frame lip I will glue it to the backing.
Step 9: Finished Product and Tips
So sometimes I'll use the strings criss crossed in an x pattern so that it will hold it better instead of straight across. I did this on the rocks in the square frame
If I am trying to put something heavy in the box I'll put it on the bottom and use the string to more or less stabilize the item, but when you put the frame on its sitting on the lip of the frame. like the salt shakers above.
Most of my boxes seem random because I tend to organize them by who the items belonged to.