Do they have a perch on a scale then zeroed to get the added weight? That seems doable.

Patient cooperation comes in varying degrees

My spouse does x-ray and they have contraptions for less cooperative humans as well!

"The Brat Board"

I've never heard her say if this thing has a nickname, so I'm going with "Toddler Tube."

Both look as equal to or nicer than how a bird gets x-rayed:

While some of the methods for handling the animals don't look friendly, the overall goal is to get them treated quickly, while harassing them the least amount possible, and not letting them get further injured then they are. Immobilization is helpful as the animals can't communicate if they have something wrong with them that we can't see until some of these tests are done.

Stress is a huge killer of injured birds and other animals, and the rehabbers will suggest not talking or playing the car radio if you're bringing in a wild animal patient, so despite how it may look at times, the rehabbers are doing whatever is in the animal's best interest. These times are when we can't humanize them, not as in don't treat them humanely, but as in we shouldn't humanize how their reactions look to us, because a lot of what might further traumatize us in a similar situation of a stranger grabbing us with unknown intent, such as covering their head, will typically calm down bird patients, while it might make a human more frightened.

It's one of the things I'm still working on is being more decisive in my maneuvers when grabbing and picking up different animals. I worry about hurting them or upsetting them, so I try to be slow and cautious, but that comes off as me stalking them to attack them a lot of the time. For many of the animals, if you just, bam, grab them, you catch them off guard and they will go into a "freeze" response and hold still to see what is going on rather than choosing a "fight" or "flight" response.