I doubt you are limited to Keeppower, but the rest of what Owen says is correct.
You want a cell rated for 3.5A (or higher) continuous discharge. For a protected 16650 this is high, but not unreasonable. If a cell doesn't have a max discharge rating, you don't want to use it in high discharge applications.
The maximum discharge is a current. A funny thing about rechargeable batteries is that there are two ways of talking about current. There's amps and milliamps (A and mA) which every field of electrics and electronics recognize. But in rechargable batteries we also have Capacity (C). The simplest way to think about C is to take the hours off of the capacity rating. So for a 10 amp-hour battery, C would be 10 amps. For a 350 mA-H battery, C would be 350 mA. 0.5C would be half the C value, 10C would be ten times the C value.
So if you had a 2000 mA-H rated cell with a max discharge of 1C, that would be 2000 mA or 2.0A. This cell would not be suitable for the 3.5A application. A 1500 mA-H cell with 3C discharge would be 1500*3 = 4500 mA or 4.5A rated, and thus good for this application.
You also want a quality, reliable cell. Bargain cells typically fail to meet claimed performance measurements, among which would be safe discharge rates.
EDIT: It appears you already figured this out. Maybe this will help some who haven't.