I am under the impression that laptop coolers don't really do much, you're probably better off trying to clean the laptop fans and if your vents are on the bottom you should find a way to make some space for them by elevating the laptop
Now that I'm thinking, the last time I remember having a laptop that overheated was running on a Pentium M series, not even Linux saved it much from the age of it x)
I used to have an Asus aspire that would barely run games and overheat in less than two hours, I abused the crap outta that thing. Ended up getting one of those laptop coolers, had one fan in it and some sort of aluminum top. Really the cheapest thing you can find, and it did the trick. So I'm thinking you might be better finding another solution to your issue, something like cleaning the fans, or re-applying some thermal paste if possible.
Just raising it helps some, but in general any newer laptop draws air from below and exhausts it from the sides so a cooler on the bottom doesn't do a lot for the laptop.
The exception is if you are resting it on a soft surface that could block vents, like your f-ing lap! In that case any of those coolers will make a big difference in both your comfort and the laptop. On a good hard surface that will make very little difference unless your laptop vents are clogged up as others have said.
A lot of them don't do much and when you do find ones that work well you have the secondary problem of finding one that lines up well with your laptop's own cooling. I just don't bother, I get a stand that allows good airflow to the laptop's vents and call it good enough.
100c means you are either running at the most it can do, Orr more likely it's thermal throttling to protect itself. Generally north of 85 is the concern area.
I would clean the fans and vents if you can, and make sure if there are vents on the bottom of the laptop that anything you put it on, be it a table or a fan, is solid. No soft surfaces.
I doubt you'll do any actual damage to it, the inbuilt safety is pretty good these days, so it's mostly just comfort and leaving performance on the table. If you can find the fan plates in a store to try them and see how much air they move, that would give you an idea as to which one is worth getting. You could also just point a pedastall fan at it, normally worth a couple degrees as long as it isn't right against it trying to blow in an out vent.
@[email protected] made a great point aboutcleaning and ensuring that there's enough airflow coming in from the bottom intake fans on the laptop. By design, laptops are designed with small spaces, so airflow is really difficult to accommodate for unless it's designed with a good heatspreader, and extremely fast fans that pump a lot of air to the critical components.
One thing that hasn't been mentioned though is undervolting.
I'd recommend looking into undervolting your GPU in order to have it work a little less hard while still getting the same performance. You can use tools like MSI Afterburner to undervolt your GPU so it consumes less power, and thus generates less heat. You might find that you'll still get the same performance, fps, etc., but perhaps your PC will reach 90c instead of 100.
Undervolting is perfectly safe, as when you use tools like MSI Afterburner, it's not designed to override the safety mechanisms built into the GPU. So if you have an unstable undervolt or unstable overclock, your computer will just crash - a simple reboot of the PC fixes that problem and then you can fine-tune the undervolt until it's stable and doesn't crash.
I undervolted my GPU for games like Overwatch, so I can play with maximum graphics settings and my GPU is on average 8 degrees cooler. Granted, I built my own desktop so I don't have real-world experience using a laptop for gaming.
Edit:
Here's a good video to help introduce you into undervolting.
I used to have some generic cooling bad which I bought on eBay for around £15. It was pretty similar to this. It worked best when I had it at about a 35 degree angle and had the laptop aligned so that the cooling pad blew air over the laptop exhaust. This would both increase the amount of cool air that the laptop could suck in, and displace the hot air near the exhaust.
My laptop would also regularly hit 100c and then thermal throttle itself but the pad definitely allowed me to get a few more frames out before the throttling kicked in.