What hinders you to adjust the current Caps to your wishes?
I sometimes generare a "clumsy beast" by simply adjusting the values of an existing cap I won't build anyway.
That usually gets ~100-150Logistic points, ~4 times the original build cost with Metal being raised the most, long build times. I mod it to be very slow (50-100 usually).
It gets ~ 1.5-2 times the defense values (armor, shields) and is being made very slow (~half the speed and 1/4 the acceleration)
I give it a VERY powerful short range weapon (range ~1500) and a slightly weaker than usual very long range weapon (~15000 range) and usually a defense against strikecraft.
The idea is a ship that comes useful at late game and that is - due to low speed - a strategic weapon which can't easily be relocated.
I always think about the "Monitor" class from David Webers book there : big, slow, nasty 