This is an insert that can be 3D printed for gap closure assays if you want more consistent gaps than those used by the pipette "scratch" method.

These have a little bit of clearance, designed for 14 mm wells. This could be used to create moulds to create silicone/metal or other material inserts suitable to be autoclaved if you want a reusable one that can be easily cleaned. Alternatively, this can be directly used - I recommend sanding the bottom so you can be sure it will lie flush with the bottom of the well-plate.