This mold uses an insert but is otherwise plain and simple.

Further information and other casting projects: http://www.thingiverse.com/Terminus/collections/my-metalcasting-projects

Update: Made a new one and poured as cold as possible but it too suffered (fewer and smaller) pockmarks. I can think of a couple possibilities. The cross is too thick for the sprue / gate / runner system or perhaps the filament has moisture in it, and steam is causing it. Oh well.. this is new territory for me and hopefully time will tell.
I smoothed and buffed it up a little but some marks went too deep. Still not too bad looking imo. Actually I think the original one is sparkly and kinda nice in its own way... just don't let on it was a reject.
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This is a very simple cross mold. 18grams ABS, 70 minutes with 0.20mm Z layers on an Afinia. Minimum fill.

If you see a lead photo of a casting with pockmarks it is because I haven't printed and cast the new one yet. I think it is illustrative to show casting defects because they often happen and can be difficult to diagnose. This one was not. The melt was too hot, poured at ~510F.
Next pour will be something around 480-490F. (R-92 pewter M.P. is 466F.)

The ABS-mold faces of the cross overheated, gassed off and bubbled up before the pewter solidified. The gases generated left the little pock marks in the pewter. The 0.2 inch thick "section" of the cross took more time to cool and solidify than was anticipated.

Another indication of a problem is the two sides of a small, flat mold like this usually come apart easily. In this case they were welded together where the hot metal enters at the sprue and had to be broken apart.

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Aside from the defects, notice the small steel pin "insert" at the top of the cross. It provides a hole for a string or chain.
One nice thing about pewter is such straight, round inserts are easily removed. A twist with a pliers breaks it free and it can be pulled out. I've had 3 inches of steel rod (coat-hanger wire) embedded in a casting and it was pretty easy to pull it out.

BTW, that pin is from a thin coathanger. Diameter is 0.075 inches. A short piece, perhaps 1/2 inch is cut and laid into the groove before assembling the two halves. You might drill the hole larger after it is cast, or with a little file work on the printed mold, might use a larger pin.

Soldering a finding is another option. Use some low temp solder. I've not done that and don't know what kind of solder to use. I'm certainly no jeweler. My pewter designs thus far have been quickies and posted for the casting exercise rather than as presenting finished designs.

Notice the mold halves were not clamped together for the pour. Instead they were cemented with some ABS cement. The reason is excess pressure from clamps can squeeze the hot softened plastic and possibly deform a hot casting. A few dabs of cement around the edges seems to be adequate.

Another point is the runner / sprue system. The melt is first directed somewhere below the casting and then rises in a controlled manner through runners / gates, and into the main cavity. This is done mainly to reduce turbulence. I don't want hot liquid metal plus air poured straight into and splashing around the main part cavity.

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Safety is everything. Although casting pewter is relatively tame there are still lots of ways to get hurt, hurt someone or something. Do not cast metal unless you know and follow all applicable safety precautions.