Is placing wooden pieces, 50 cm long, in the mosque for use as a Sutrah for worshippers in optional (Nafl) prayers considered an innovation (Bid'ah) or a matter of convenience and facilitation?

Placing what worshippers take as a sutrah in the mosque for fear of people passing in front of them is not considered an innovation (bid’ah). Rather, it falls under the category of assisting in obedience and facilitating the performance of the Sunnah. This is based on the saying of the Messenger of Allah, peace and blessings be upon him: "When one of you prays, he should pray towards a sutrah and draw near to it."