Crossing your back is the best. You don’t want to mess up the front face of the sex doll. The scars may be more messy than you think. You should post a skull picture here so that other TDF members pay 2 cents. Try to simulate what you want to do, take some channels and test them to see how it looks and how easy it is for your ideas to come together.
This is a problem with many of these doll sub-frames … many are outsourced because there are many similarities in design. Some people now make child frames internally. This is the difference between TPE and platinum silicone resin. One is a hot casting process and the other is a cold casting process, so you can avoid the use of PVC metal joints wrapped with foam.
When joints break, this is a very simple fix for sex dolls. However, when using TPE, you may encounter problems if you weld metal fittings to pipes or rods. Then you are working on a whole new set of questions. Here are some good things to fix tpe with a hot air gun, I think if we cut our back for spine repair or replacement, we will steal some tpe from the inside and replace it with foam.