Many different materials can be used as an absorbant...
I find Long grain rice to be a poor absorbant in the typical cooking time of a meatloaf, "Quick" rice not much better, but "instant rice" seems to work ok
Oatmeal works very well as do corn flakes, I don't like using saltines for meatloaf
(but they are preferred for Salmon or tuna casserole due to their neutral flavor)
Oatmeal tends to help bind the meatloaf together without using additional eggs.
If there is a lot of fat draining out of the meatloaf you either need to use chopped
meat with less fat or use more absorbant.
Another absorbant you can use is instead of chopping fresh onions use dried minced onions, they are "hungry" for moisture.
If the meatloaf is too crumbly add more eggs
The thing to remember is that a meatloaf or a fish casserole are
in essence a "savory meat custard".
How do I make it? Highly variable depending on my mood and available ingredients
as well as what I plan to serve it with.
I've used Beef, Venison, Bison, veal, ground pork, even ground lamb,
as well as mixtures of most of these..
One of the best was Bison with partially cooked bacon and carmelized onions
mixed into it.