Product development (mechanical engineer / design strategy analysis)

I am working on a product design concept. I would like to get to a point where I can prototype and obtain manufacture / moulding costing.

In order to move to the next stage I am looking for the support of a mechanical engineer to help identify the simplest/cost effective solution to allow for the "plastic box unit" to feature an expandable winged shelf that can be; split collar locked, clutch lock, snap locked into position to stabilise its position.

I am looking for ways to simply/cost effectively build this into my mould / product assembly.

Thank you