Short answer
Send four things: a photo of the rating plate, a photo of the grill seen from below, the manufacturer's installation page if you can find it, and the outside dimensions. That combination is normally enough to establish the aperture.
Start with the rating plate
The plate carries the model, the serial and often the gas category. With the model we can check a library of about 180 built-in grills, and where the model is not there, we can usually find the manual from it.
Photograph it straight on rather than at an angle, and make sure the model characters are legible. That one photo saves the most email.
Then the underside
Photograph the grill from below, straight up. The aperture is fixed by the chassis outline and the flange overhang, and both are visible from underneath in a way they never are from above.
This is what makes an unknown model workable. The chassis tells us the opening; the flange tells us the bearing width.
Then the outside dimensions
Overall width, overall depth, and the height from the bottom of the chassis to the counter line. These cross-check what we read from the photos and catch the cases where a grill is a rebadged version of another.
Measure the chassis, not the lid. The lid overhangs and including it is the commonest way a grill appears to need a cabinet 200 mm wider than it does.
When none of it is enough
Then we ask for the manufacturer's installation page, or you request it from their support with the model and serial. Every mass-produced grill has one, and manufacturers answer this request routinely because it is what every fabricator asks them.
The waiting is normally two or three days. That is cheaper than a carcass cut to a guess.
The four things to send instead of a cut-out number
Aperture width and depth, measured on the appliance
Not the outside width. The aperture is the hole the grill drops through, measured to the nearest millimetre.
The flange or rim width
The lip that carries the appliance. Without it the aperture is a guess, because the hole has to be larger than the aperture by that amount.
Lid clearance, including the swing
Height above the counter when closed, and how far back the lid travels when open. Under a roof this is the measurement that decides the layout.
The gas entry side and the hose length
Which side the hose leaves and how long it is. That fixes where the bottle has to stand.
Questions we get about this
What if the grill is a one-off or locally made?
Then a physical template is the answer. Cut a template to the chassis outline, send it, and we cut to it. A one-off grill with no documentation is exactly the case where a template earns its postage.
How accurate do the photos need to be?
Legible, straight on, and with a tape measure in frame if you can. We are reading shapes and proportions, not measuring from the photo.
Can you cut to the grill itself if I ship it?
For commercial orders we have done it. For a single cabinet the freight both ways usually costs more than the risk of being 3 mm out.
Before you order anything
Send the grill model and the run length. You get the aperture, the module list and one itemised price back in writing — freight shown separately, valid 15 days.