I tend to do a modified pushdown instead of regular skullcrushers since there's no resistance at the top of the movement while laying flat.
Instead of doing the pushdowns standing up with upper-arms fairly vertical, I'll bend forward just a bit at the waist and instead of having upper-arms mostly vertical, I'll bring my elbows up a few inches so my upper arms are about 45° in relation to my torso (or the floor, if that makes sense), & I'm pushing "out" as opposed to pushing down. That way I'm sorta doing a movement similar to a skullcrusher, except i still have resistance while in the fully contracted portion of the movement.
I also like to do overhead extensions while laying on a 45°/35° incline bench using a cable... Again, do to maintaining resistance throughout the entire range of motion. Similar to the movement I described above, except the upper arm is at this "\" angle, opposed to that "/" angle described above (obviously).
I generally prefer to begin my triceps work with arms overhead in an effort to engage the long head as much asnpossible while decreasing long head recruitment while progressively increasing lateral head workload.
Sorta like overhead/straight up "⬆",
then overhead 45° "↖"...
...However, I do conventional pushdowns next "⬇",
then to modified pushdown/skullcrusher-substitute "↙", because I can't use as much weight on those. (well, I "can", but other muscles get brought in to play at the "top" or stretched portion of the movement... Such as lats, rear delts, serratus, & abs)