This involve using keyboard macro application of your choice.
Most gaming keyboard come with one.

---

Bind to a key of your choice or override the default one with two key in sequence:

1. Sprint key down.
   
2. Crouch key down.

Don't put in the released up key event in if using tap crouch mode in the settings.

When using the macro and not pressing W; you will crouch.

Now you just have to hold W key and press the slide macro key when on the ground for an instant slide!

Additional tips:
When in the air, listent to your foot landing timing or concentrate on the ground for when to press slide macro key. Eventually you with get used to the timing.