We are busy printing respawn boxes for my other business, Laser Commando (lasercommando.com.au) and we need to bolt things together. IWe tried to bolt into the plastic, but it kept on splitting. So we decided to print with nuts in the holders. I will glue the holders into the respawn boxes and then I can bolt the back panel on.
We use this method now consistently to add nuts into printed parts. It is very easy to do with Gizmetor. Use the preview manager and scroll through the layers. When you find the last layer just before the nut holder area closes tell Gizmetor to stop at that layer. The printing will stop and then you can add the nuts. When the printer continues it will close the nut and you can then bolt your plastic parts together.