A panel that feels rigid on a workbench can bend noticeably when it projects beyond a mounting bracket. The unsupported length, load position and restraint at the bracket all matter. Specifying only thickness leaves most of that problem unanswered.
Measure from the support, not the outside edge
The working span begins where the panel is actually restrained. A bracket may overlap a large area while contacting only a narrow strip. A soft pad, loose fastener or flexible supporting frame can change that restraint again. A sketch showing the contact region and the load location is more useful than a thickness request made in isolation.
Why a longer overhang is a different design
For an ideal, uniform cantilever beam with a point load at its free end, the elementary bending model makes tip deflection proportional to the cube of length when the other terms remain fixed. That is a useful explanation of why small changes in overhang deserve attention. It is not a direct sizing rule for every composite panel: laminate direction, panel width, cutouts, joint flexibility and the actual loading can require a more suitable analysis.
Put the allowable movement in practical terms
State where movement matters. A tray carrying a sensor may be limited by alignment with another component, while a cover may only need to stay clear of a moving mechanism. These are different requirements even if the applied force is similar. Give the relevant load case and a measurable displacement limit at the point that affects operation.
What to take to a supplier review
Bring the mounting layout, support construction and proposed load locations together with the panel outline. If a first sample will be checked, agree the loading arrangement and measurement point before interpreting the result. A hand-held bend comparison can be a quick discussion aid, but it cannot replace a recorded acceptance condition for a repeat order.
Technical background: MIT OpenCourseWare: cantilever beam example.