Been there, done that. My tips:
Cruise the RV and boat parts sites to see what hatches are available, may be visit a view local stores. Many have calalogues, and will order just about anything for you.
Look at the trailer and carefully decide exactly where the hatch goes, what shape, and what size. Remember you should plan on installing a wooden back-up inside the trailer for support, especially with the larger, rectangular hatches.
Do not cut anything until you have the hatch in hand, have put it up to the trailer, and you are sure it is what you want.
Measue the hatch carefully to determine the correct hole size.
Decide on a hole starting point (corner of a rectangular hatch, bottom of a round hatch, allowing for the flange), and drill a small hole through the shell tp the outside from inside the trailer. This will ensure no mistake on hatch alignment.
With this starting point, marke the rest of the hole pattern (circle/rectangle) on the outside of the trailer.
Cut the hole, staying inside the lines you made. It is easy to enlarge the hole if needed, really tough to make it smaller. I used a metal blade in my jigsaw.
Install the wood back-up within the trailer. I also fiberglassed the lower lip of my wood backup piece.
Install the door using the soft butyl caulking compund used in trailers. Be generouse here, and tighten the attachment screws evenly across the entire frame. The butyl caulk will squeeze out over several days. Take a popsicle stick, and sharpen one end to a 45 degree blade. SLightly dull it on fine sandpaper, and then use it to "cut" the caulk away from the trailer and frame. Incredibly easy to do!
Final FYI. Go to your locksmith and buy a new lock cylinder for the door. For some misbegotten reason, the RV industry only uses two key patterns for these small locks, A and B. Not sure, take you key to a dealer's lot and see how many compartments you can open!
See my photos: