Can you easily jury rig a bump stock instead of buying a real one?

You can 'bump-shoot' a gun without a bump stock. You just have to know how. The bump stock makes it easier but you can still do it with a little practice. You hold the gun low, in a position where it can move back and forth. When you shoot it, it jerks back and then comes forward again, and if you have your finger against the trigger when it moves forward it will fire again. Just like a real machine gun, you get a lot of lead in the air at the cost of accuracy.

The guy in Las Vegas, I don't think he even had a bump stock.

You didn't really think Pres. Trump would go for any restriction that would change anything, right?

no

The notion of bump stock was to reduce the impact of recoil, like shock absorbers on a car.

This allows a lot of movement and holding the trigger finger firmly allowed the trigger on a semiautomatic to fire with recoil action. Any seasoned shooter could do the same without a bump stock, but makes it much easier.

My take is the notion of such rapid fire is pointless, that sort of fire is fine if you are hosing down a wide area with bullets, but a real waste.

The need for firing several rounds in rapid succession, a bump stock will hold a better pattern, so will good recoil pads. I have shot bump stocks and conventionals, and see little benefit other than combat.

Makeshifting would not be worth the effort, Need a high rate of fire, go with a full automatic. Never had the need. To me, a machine gun is a nice conversation piece, nothing more. I don't expect combat.