Old steamer trunk restoration

[overtime]

Hi Ive purchased an old steamer trunk for hobby , Ive limited knowledge on woodwork but am lost on this one. The trunk has a hole small and its a little brittle around the edges to remove the dodgy stuff it would be 6 x 6 inches , It appears that the material is a type of plywood with some type of covering(?) Ive struggled to access appropriate info on the web mostly they refer to hardware replacements no mention of how to fix the hole. The only feedback I received was advising me dont remove the nails and replace the whole panel of the lid rather cut a square section out and replace it with a same size panel . But no mention how do you attach it to the lid , I cant see how it would sit and by what means you would fix it without loosing the strength integrity of the lid ,
Can anyone advise me or direct me to information on this matter please

[JPG]

A pix of the holey trunk would help.