The general rules for extended travel (more than an hour or two) are:
- The dog should be able to stand up with the head held in a naturally high position.
- The dog should be able to lay down straight without having to curl up or draw any legs in.
- The dog should be able to turn around 360° in the travel box.
- There should be an attached water bowl.
For longer travels (more than 6 hours) these additional rules apply:
- The travel box should have an attached food bowl.
- The travel box should have an absorbant inlay to absorb urine.
- Ideally the travel box is big enough that the dog can avoid stepping or laying in its own feces.
Now, these rules are individual to each dog. So if your dog bumps its head on the box when sitting or standing up straight, the box is too small for your dog, but it might fit different dogs. It might be ok to transport your dog in a box that's technically too low for 2 or maybe 3 hours, but anything longer than that means increasing discomfort because the dog is either forced to remain lying down or its head is pushed down, which can lead to sore muscles and probably a headache.
I also understand that it gets more and more complicated to follow these rules the bigger your dog is. Some airlines simply won't accept travel boxes exceeding a certain size.
BTW, there is an international standard regulation for transport containers of animals that most airlines adhere to:
- It includes size requirements in relation to the actual size of your animal (pretty much identical to the one you posted).
- All doors and/or gratings must be secured in a way that the dog cannot possibly push them open or hurt itself while trying to open them.
- The travel box must have grated openings that allow air flow on at least 3 sides.
- The travel box must be surrounded by spacers that avoid the blocking of the ventilation openings by other objects like luggace containers.
So it should be possible to build your own travel box according to these specifications or modify a different container to fit these requirements. It's always advisable to ask the individual airline if they accept custom transport boxes for animals.