Getting dice to be perfectly proportioned is difficult. I doubt that I could do it.  And there would probably be a weight issue, too. If one side was more dense than the others, you would have 'loaded' dice.

If these are just for show (not use), the only thing I can think of would be to use a mold. Find something that is exactly the right size, and make a mold of it.  Tricky.

Sue

The best solution is to make your own plaster molds. Either make  original cubes from clay or use wooden or plastic cubes. You'll need a release agent.
Google 'paper casting' and 'molds' or (British spelling) 'moulds' and you should find tutorials.  You might find some ice cube trays that are the right size, but don't think you can cast paper in them.
Good luck!
Jonette

I didn't see any mention of how big these dice (die, dices) are.  Since you mention plastic containers I guess they are fairly big.
If so, casting would be difficult for, as Sue said, you can get distortion.

What about another approach . . . . cut squares of strong cardboard to the size you want.  Dribble PVA along the edge with a small gap at ends and middle.   Join pieces by dabbing hot glue in the gaps and immediately pressing them together.  The hot glue will take hold and allow the PVA to dry more slowly.  Cover with a layer or few of paper laminate.
DavidO

I haven't cast papiermache in about twenty years, but used to use dishwashing liquid soap for a release agent for molds I made from clay objects. Somewhere here I've got my pm books with info on release agents -- I'll have a look and get back to you asap.

Jonette

I just thought of something that might work. Since the dice I'm making are solely for decorative purposes, perhaps I could make a cube shape using cardboard and put pulp inside of the empty cube so it won't be crushed.  I would have perfect edges, it would still be light, and I don't have to spend money on buying a ton of dice.  Do you guys think this method could work?

May be I have a solution for your 
                                             
                                             
problem.
You can take a little square piece of paper.
Fold 16 squares,like in 
primaryschool.
                                Each number is a folded square.Because I can't draw
                                here, I just placed the numbers.
                                Remove square 5,9 and 13.(scissors)
                                Then cut between square 2 and 3,
                                             square 6 and 10,
                                square 10 and 14               

                                             between 4 and 8
                                             between8 and 12
     1  2  3  4                                        between 12 and 16
     5  6  7  8
     9 10 11 12
    13 14 15 16                        Now you can fold a little box with
                                           
a lid.Glue the sides,fill the box with sawdust or rice,glue the last part and there is your dice.
Stiffen with PVA or paverpol,paint ..and ready.
I wanted to type this very nice,but I am better in folding .I am bad with computertyping.














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