If using glass for your mold (+high Tg resin), what I would try is to lay the fabrics on the particular orientation that avoid or reduces the expansion. The expansion should be mainly on the longer side direction. (lay fabric diagonal +/- 45 to that direction. Avoid fabrics aligned. The longer the fabric, the bigger expansion (linear variation) This may help.
In a begining I would post cure in-mold. The resin can contrat a bit after heating. If the part is free, the tenssion owed to the contraction can distorsion the original part´s geometry (commonly "closing" the U shapes, and similar, not only linnear contraction)
IF POSSIBLE I would try a first curing of the part. Once cooled, with a good resistance on the part, I would do an in-mold post curing.
On the technical data sheet you can find the amount your fiber elongs per ºC. Compare and choose a low expansion fiber (idealy carbon) and combine with orientation.
The mold must be made with a good enough resin (high enough TG). Maybe you can postcure the mold itself (with your plug, again to avoid distorsion) to reach the highest possible Tg. Check resin´s Tg Vs curing cycle graphic on the resin´s TDS.
