Hi Denis
Not sure if this is connected to your other post regarding the instrument box but it sometimes helps to tackle difficult corners by using separate pieces of fabric, especially where there are multiple curves and vertical sections in the mould
You can create templates by sticking down masking tape in your mould sections, drawing the lines you need to cut on the tape, then carefully removing the tape and flattening it down onto something like cardboard to create cutting templates
EC's Profinish is a good material for this but applying spray tack to the inside surface of your fabric before you cut can also help you achieve neat cuts for joints or overlaps. I would also recommend a good roller cutter which can help you create nice straight lines.
A joint made in the radius of a corner can often be well concealed if you want to avoid pulls in the weave pattern from trying to use a single piece and, for very small sections it might also be worth considering a lighter fabric as often used to create phone and iPad cases