PETG need to have the starting height set correctly. What this means is for whatever layer height you are using, you must adjust the Z offset so the part comes out the correct height. When set correctly and the PETG is a quality product and dry, and your starting offset is correct it sticks like glue and does not warp. I am currently printing some finger gards which are 7" long and 4 inches wide and 1.5mm thick using PETG with not warping. One thing to note when designing part for 3d printing, using fillets to round corners and edges is really helpful to stop warping.
If you tell me your nozzle size I will upload a model that is 3mm tall, that you can print and measure the height of the walls with an accurate digital caliper. They should be 3mm tall +- .02mm
If not adjust with the z offset in your slicer or within the firmware depending on your experience level.
Hope this helps
PS PETG is not stiff it is very flexible, for parts that need to be accurate and stiff I typically use ABS which is a bigger can of works with warping and difficulty in printing.