The last and the essential measurement is for the mounting hole for the Speaker.
The mounting depth of the Speaker has to be measured both externally and internally. To measure the exterior, have a ruler, insert the hole to the maximum limit, and mark.
If the mounting hole is non-round, you have to measure the internal diameter of the mounting hole. Have a ruler, place it on the one recessed hole’s side, and extend it to the other. The cutout size has to be a little smaller than the recessed slip of the Speaker or the speakers will not fit in the hole.