The L-sit is a true testimony of core strength. Sadly, most people who can successfully do a gymnastic L-sit are, well, gymnasts.
Remember that elite performance usually comes hand in hand with a body type that's best suited to that endeavor for every sport. There are high-achieving outliers.
The ideal body type is typically compact for gymnasts, with a longer torso and shorter legs. However, if we want to recreate these patterns to cater to a taller, long-limbed dude who's got a whole lot of meat on his bones, we have to get innovative.
For the L-sit, we have to acknowledge that our legs are just too long and heavy for us to coerce physics to join our side. Two modifications/progressions will help.
The first one is simple. Attach a band across two low safeties in the squat rack. Place your heels on the band and lift your body. Keep rigid legs and pointed toes. Don't "rely" on the band, though. Do everything you can to lift your heels up, away from the band. Pretend you don't want its help.
Attempting holds of 20-30 seconds is a good place to start. Once it gets easy, don't hold for any longer – just use a skinnier band.
A second progression is wrapping a band around yourself rather than having a band help lift your heels. First, the movement becomes harder. Since your heels aren't "resting" on the band, your hips, quads, and core are responsible for lifting the legs.
Note that I'm not wearing shoes in this variation. This keeps the band from slipping, but even more importantly, it forces me to point my toes, which is crucial to doing proper L-sits and a good habit to adopt so that you totally brace the anterior chain.
Though the force angle is a bit different when the bands are around me rather than below my heels, it still offers a bit of assistance to make the lift possible for a 6'4" lifter with long legs.
The goal is to use progressively skinnier bands until one isn't needed. Since this version's harder, start with sets of 15-20 seconds.