The standard anchor nut for the model C isn't great. This one takes a square nut which is much easier to fixate.

The original anchor for the model C has a nut-hole that is too large for the required M3 nut, and will spin around if you don't glue it in. After fully assembling the body I came to this conclusion which was a pain to fix without throwing the whole body into the trash.

Be kind to yourself: get some square M3 nuts and print these nut anchors instead.

Press the nut into the hole all the way with a screwdriver. It should be very tight in the hole. If you have a hard time just scale the model 2% in the direction that doesn't fit for you.

Print this model with the large flat side facing down, the sidewith the cross cut-out. No supports required.

FYI This model is for this (great) model airplane: https://www.eclipson-airplanes.com/modelc