The critical measurements.

This stage of the customization is critical. If you get it wrong here the whole project will go wrong.
So plan with precision, measure ten times and cut only once. Aways cut long at this stage leaving room for ajustments. This type of work must be finessed not pushed and bashed. Think of all the possible ways it can be done. Once you have a clear picture to what you believe will be right get started. Remember you can be flexible with your ideas, these projects have a feeling of being right. There are no wrong ways only results. It may not be the result you like. If not change the way you are going. These projects are a work in motion and always changing.
Don't get caught out thinking you can do an engineers drawing and do a project like this. It will never work. Just get started and it will flow.