material for ceiling access cover plate?

Any suggestions please for a material for ceiling cover plates for maintenance only access to a junction box and 2 drain taps in the H&C water pipes? The floor above is (deliberately) completely sealed and intact so access must be from below. The rest of the ceiling will be finished in skimmed 12mm plasterboard.
The cover plates need to be 400mm long x 75-100mm wide and ideally should be paintable and merge in as much as possible with the rest of the ceiling.
Is there anything better than 12mm plywood?. Or any specific products made for the job?
TIA
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