I've been told that tone is in the fingers. You have to use double bass fingers on an electric bass.
1000000%
The sound of an instrument is more than just its "tone," IE it's harmonic content.. The sound also has a physicality, where in the listener is told things about the object, making the sound in the process of the sound creation (Stopping well short of the tone-wood debate, lol..). The sound has artifacts from the object's construction and playing technique. The sound has an envelope and either it decays or can be pushed through.
I DON'T suggest making the rule of thumb "emulate double bass technique" BUT the very close relative of "inform your technique with the physical data from double bass technique" is absolutely money.
The little 'whiff' that comes from the common upright technique of dragging the inside length of your index finger is something we need. Playing with that kind of finger movement works well.
Very important is the left hand. Playing positionally with a lot of deliberate and precise shifting, rather than using 4 spider fingers bridging whole steps, is going to lend itself more to the vocabulary and play style of an upright player. Another thing you can recognize from Gustav's playing is a very fluid combination of being
informed by the left hand idiosyncrasies, while taking advantage of what a bass guitar needs from you itself.
This is absolutely one of those things where all the gear in the world won't get it done, but for once it's also something that does need some gear considerations, if only the mute.
One more little thing in terms of the physical artifacts of instrument's sound- I said I dislike the Tapewounds I've tried for the proxy double bass role.. but one thing I LOVE about them is the particular thwack sound they make both as fret noise and right hand 'bite' . It's a fantastic addition to the upright sound, because counterintuitively I think that very low action is actually better for pulling out the sound to mimic upright, just because of the frequency of string/board contact and because is allows you to really pull the right hand with the claw without being able to over do it.. with tapewounds this sound is absolutely great, but the ones I've tried were too floppy. I may re explore this with the ones suggested above