I have been keeping my eye open for a decent bar clamp to make a stock mono pod for my Impact for several months now and recently ran into airgunbillls post
https://www.airgunnation.com/topic/a-couple-of-homemade-gadgets-for-my-fx-impact/ He made a pretty decent monopod but I had something different in mind.
His post has information on the Smallrig parts and sure enough they had exactly what I was looking for a nice NATO rail rod clamp. So I ordered one here
https://www.smallrig.com/smallrig-swat-nato-rail-with-15mm-rod-clamp-parallel-1254.html and a couple of 6” 15mm carbon rods to go with it.
https://www.smallrig.com/smallrig-15mm-carbon-fiber-rod-150mm-6inch-1872.html 
These are the parts I purchased to make the monopod. A Smallrig NATO rail bar clamp and a carbon fiber rod to go with it. Sorted through my junk box and found a couple of picatinny rails that would work.
I decided to cut the longer rail and use it as it matched the rails already on the gun. I drilled another hole in the rail for the original Impact butt pad adjustment knob screw to go through to hold the bottom of the rail but found the original Impact screw to be to short. So, I purchased a longer metric Allen screw, actually ten of them, M5x.8x35mm to use with the original knob. I slightly turned down one of the T-nuts from the other rail I already had and used it to mount the top of the rail..
The knob had locktight applied but came off easily with an Allen key and my fingers. Put a drop of blue locktight on the new longer Allen screw and screwed it into the knob.
Installed the T-nut on the inside at the top, with a screw, and the stock knob, with longer screw, on the bottom to hold the rail.
The NATO bar clamp slides right on the picatinny rail once adjusted correctly and the carbon bar slides in the clamp as well.
This is a simple inexpensive, about $50, monopod mod and the install looks nice and clean. It can raise the rear of the gun three inches when installed this way or five if the NATO rail clamp is installed below the knob. It is a quick release type of rail clamp so the unit can be removed easily if one chooses. The butt pad adjustment knob still werqs to make the butt pad adjustable although I may change it to an Allen bolt so I can just slide the clamp up and down the full length of the rail. I still need to put an adjustment knob on the bottom of the support rod for fine tuning as well.
No holes were drilled and no permanent changes are made to the gun. It is easily returned to it's stock form.
www.airgunnation.com 
