First of all, welcome to BPO Thelma and Duke!
As far as the arm for a diy grooming table goes, I would make it out of PVC and based on the way they do adjustable weights in the gym.
First, I would gather up my supplies:
*3 straight PVC tubes - one slightly larger than the other two (the smaller one should be just one size down from the larger one so it can just fit inside)
*One L shaped PVC piece to connect the two smaller straight pieces
*A pvc cap that fits onto the smaller pvc tube.
*A drill - I suggest a drill with a spur point drill bit, say 3/8" or so
*A bolt that is as wide as the holes you drill. The length should be the width of the wider pvc plus enough room to add a nut to secure it.
*A cap nut or acorn nut
*A sturdy eyehook with a nut that fits it.
To put it together:
*drill holes at a fixed interval through one of the smaller pvc tubes. They need to go straight through the center of the tube
*drill a small pilot hole through the center of the pvc cap, insert the eyehook, and scure very tightly with a bholt.
*Connect the other smaller pvc tube to this one with the L shape, and place the cap on the end of the one without holes.
*Drill a hole a little bit down from the top on the wider pvc tube. It needs to go straight through the center of the tube
*When the smaller pvc tube is placed inside the wider one, you should be able to adjust the height by lining up the holes so that the smaller one goes as high as you want it to. Then you will secure the two pipes in place by using the bolt and cap nut. Whenever you want to change the height, all you will need to do is remove the bolt, change the hole that is lining up with the wider tube, and then replace the bolt.
How long you cut the tubes to be depends on your needs.
It will look like this
Then you need to attach the wider PVC tube to the side of the table. Then you can either buy a noose or create one and hook it on to the eyehook
I am not sure if I explained that well - it makes sense in my head, but if it is confusing written down let me know any questions you have