[One of Many]

I doubt it is the wood specifically. It sounds more like nesting error. If the dish shaped part is shifting on the plug it sets on or in, it will show up in the depth and location of the groove in relation to everything else.

You mention this groove runs around a cutout. Is this molded in or is it a trimmed cutout? My thinking is that you need to take advantage of whatever is consistent in the parts "as molded" features or request an indexing boss(or 2) be added to the mold in an area that gets trimmed away. As long as those indexing bosses are used on your fixture, all should have better nesting repeatability. Multiple cavity mold tools the parts are formed on can also add to the problem if the molds are not dimensionally identical.

Selecting a better sequence of the trim operations, like cutting the groove prior to trimming the cutout may keep the part a bit more stable and less risk of a vacuum leak contributing to the part shifting on subsequent cuts from either vibration or cutting forces.

I did a lot of this in vacuum formed plastics. We used MDF or Benolex and Bondo or Urethane filler as a plug made from the run of parts in production. The big headache I had to deal with was inconsistent shrinkage or warping of the parts after forming. Getting the part to set on the fixture and stay in tolerance could be a pain from one run to the next. If the part is going to dance around, you must find a way to make it submissive. That can be done as stated with features in the mold or secondary operations in the trimming. I would have the trimming people drill all holes as a first sequence and use pins through the drill bushings to then stabilize the part prior to any routing cutout operations. If no holes, then I would put scribe lines on the molds non-critical area and indexing reference lines on the fixtures to assist with orientation on the fixture.

Just don't expect to get any credit for resolving quality problems no one else has ever made an issue of. Crude tooling of this type does not mean that will not do the job. It often means the person using it must exhibit more skill in making it work for the job at hand. A good toolmaker should not put that part of his job off on someone else!

DC