Ok I decided to make this post because I saw people confused about how to put their battery slider back on the Geekvape Aegis Legend 2, L200 as mine fell off too a while ago.
What you’ll need:
A torx size 4 screwdriver
Id recommend a pair of tweezers to help place the screws into the holes.
It’s really simple, actually even easier than I thought it would be. You just need a t4 screwdriver, torx size 4. I picked up a small “phone repair kit” that’s got a bunch of little interchangeable bits. The L200 uses t4 and t5.
THESE ARE TINY SCREWS DONT DROP ANY ON THE FLOOR LIKE I JUST DID. 
if you drop them they will bounce away off of just about anything.
So, when you open up the battery door you will see 4 t4 screws holding down the silicone+battery contact to the bottom of the battery door. Just unscrew them. I recommend doing this over a dish to catch them in case they decide to jump out and bounce away.
Be careful when you pull the silicone piece off, the tiny t4 screws for the slide can fall out now.
(You can see one of mine did, it’s just off camera.)
Remove both of those t4 tiny screws, and put them somewhere safe.
Next, line up the slider with the holes on the bottom of the battery door. I have it upside down in this picture just to show which way it lines up.
Put a piece of tape on the slider to hold it on to the bottom of the battery door. It’s not totally necessary It just will help so you can flip it over and put the screws in.
Gently put the screws in the holes, don’t try to push them down into the holes or else it’ll pop the battery slider back off. You want to just screw them down into the holes. It’s probably best to use tweezers, what I do is push the screw onto the screwdriver to help guide it in.
but I warn, the screw may fall off the if you breath on it wrong.
Lefty-loosy, righty-tighty.
When you got both in, make sure they’re snug but don’t over tighten or you’ll risk stripping out the holes on the slider.
Yay.
Put the silicone piece back into place and screw it back in. Dont overtighten, it’s seems to deform the silicone, and make sure you have the contact tab lined up with the little contact button inside the battery compartment. Once again getting the screws back in the holes. (Without dropping them… I just did and it just ricocheted off the tool kit and went onto the floor somewhere. I FOUND IT.)
Yay.
Before my battery slider fell off the door needed a little help coming open. Not sure how to fix that, but at least now I don’t have to use my fingernail to open the battery door anymore.
