Do you know about any good 26650 flat top batteries that can be used in serial without using magnets or soldering etc.? Is it legal to combine BT and FT in serial, as long as they else are identical batteries. Ie. Liitokala 26650 BT 50A together with flat top version of the same battery? Maybe stupid question, which of those batteries should then be positioned first in the serial configuration, FT or BT?
RapidLux, yes no problem combining under the hood or otherwise identical flat top and button top cells.
With regards to which cell to put first when in series, first question is is the flat top protuberant? If the answer is no then the flat top may only work at the top position.
A low temperature solder like Rose's metal is an excellent way to button top a cell. You can buy the stuff in pellet bags on eBay for relatively little money. Also get some steel flux like this if willing to try this path.