You’re welcome. Most of my knowledge about flashlights and batteries comes from reading a lot on BLF. I’ll try to explain you a few things about batteries.
In my NI40 I use mostly Liitokala 26650 batteries. They are rather cheap with an acceptable performance. Those who have the best performance and top notch cells are certainly Vapcell, Shockli, Keeppower and some others. Here is a list of the reputable sellers where you can buy good batteries: Massive post on where to get lithium-ion batteries everywhere in the world! Look at Post 2-5 for info, And Canada Post 123
Flat top unprotected batteries will fit almost all single cell flashlights. It depends on how long the battery tube is and on the spring tolerances if a protected cell is suitable or not. Sometimes the description tells to use batteries with more than 10A continious discharge. Then they mean unprotected high drain cells.
The Emisar D4 or the Lumintop FW3A for example fit only unprotected flat top batteries. Flat top is how they are sold by the manufacturer.
Other sellers will add a protection or a button top. A protected battery is not the same as a button top battery. Most protected batteries have a button top but not all button top batteries are protected. Many flashlights with 3 or 4 batteries and battery carriers require unprotected button top cells.
In the SP36 there are 3 18650 batteries. They need to be unprotected because protected cells would be too long. They also have to be button top because there is a brass ring at the head where flat tips would not make contact because usually the positive pole is slightly lower than the surrounding shell of the battery.
You should use unprotected button tops also in the Haikelite MT09R. It has plastic rings around the positive button that prevent contact on flat tops and are a mechanical reverse polarity protection. The MT09R is a high power flashlight with 3 XHP70.2 that need a lot of current in direct drive. With protected cells it would flicker and simply turn off in turbo mode. It has all necessary protections because it uses the NarsilM firmware and the BLF members who developed this firmware and the corresponding drivers are true experts.
Generally speaking, all high power flashlights that use the XHP70, XHP50, SBT90.2 and multiple emitters need unprotected batteries These flashlights also have low voltage and reverse polarity protection. If you see lumen values over 3000 lumen you can be certain that these flashlights use unprotected cells. You can find out if flat top or button top by reading the description or reviews.
For more information about batteries, leds, drivers and flashlights in general check out this reference guide: LEDs & Other Stuff - (Reference Guide)
There are the battery tests by HKJ, the safety guide and links to useful information about batteries.
Hope I could help you. The Sofirn SP36 and the Haikelite MT09R are two great choices.