I’m somewhat of a noob venturing into more “enthusiast” oriented lights, and have a C8 on the way as well.
The most often recommended cells for it seem to be the unprotected Samsung 30Q and Sony VTC6. While there are protected versions available, most are not, and substantially less expensive as well.
For the Astrolux C8, one thing to be aware of is that its direct driver (in the high modes) will have higher current demands on the battery, which calls for a high discharge battery and appropriate protection circuit (if present). The latter will place a constraint on the current available vs. an unprotected cell.
As one example, the Klarus 18GT-IMR31, which appears to be a protected cell based on one of the above cells, is rated to deliver a maximum continuous current of 12A, a cost of 3A over the naked cell. There may be other protected cells out there that can provide the equivalent, or something closer, but I’m not aware of one.
I’m not sure what the C8 typically draws in use, but if 12A is sufficient, then that battery is a suitable candidate, provided it will fit into the body tube. Protected cells tend to be longer, and marginally larger in diameter than unprotected cells. A post elsewhere here cited the Convoy C8’s maximum battery length as 70.5mm.
There’s also the question of margin. Let’s say the C8 will draw 10A in use. Does one opt for the protected Klarus operating at closer to its maximum rating, or something more comfortable with a bare cell?
Aside from the physical dimensions, on balance, the potential compromises in performance, and margin, as well as the extra cost of using a protected cell, firmly favor the use of an unprotected cell in these particular lights.