I etched and painted the pluses & minuses on where they belong (i.e. pluses at 5 and 6 on an ordinary die, minuses at 1 and 2). They're obviously home-made, but I think they worked out well.
I did the etching by hand with a drill, and the paint is Citadel "skull white", an acrylic modeling paint.
I was curious about whether the dice were particularly biased as a result of the (shallow) etching. Presumably not, because what I etched out accounts for so little of their mass.
Here are the probabilities associated with the results of rolling 4dF (i.e. adding pluses and subtracting minuses). Blue dots are ideal probabilities for each result; red dots are observed probabilities based on a sample of 180 rolls of this set of 4dF. See the data or graphing code.
So no glaring problems there. Fun!
Finally, I was looking for a way to package these as gifts and came up with a simple folded-paper box: