Flex Watt is simply rebranded floating floor heating film (made by Calorique, which is primarily a floor heat system manufacturer). Contrary to a lot of belief, it isn't a specialty product (though the widths marketed for reptile use are indeed special). I assume there are heated floors in Sweden (I'd guess even more so than in the US), so such products probably exist there in a width that would be suitable for that size tub. It might also be that there are some North American vendors of Flex Watt that would ship overseas.
A heat mat that is the appropriate size (about a third of the footprint of the tub, in a room where the ambient temp doesn't get too low) would indeed work. Individual heat mats are expensive, and the wiring is messier (more cords), but the thin film ones are exactly the same sort of material as Flex Watt and would work exactly the same.
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