Yup, more Pipelauncher.

The pipelauncher is a pipe with one end capped.
It's purpose is to be non static launchpad- it adds
velocity to a rocket. It uses the ocean environment and could
be particularly useful if you want to launch rockets from the equator.
As I see the pipelauncher, it's main benefit in the near term is to
be a relatively cheap launchpad.

But now I will describe a pipelauncher which has more emphasis
on adding velocity to a rocket instead mostly being a launch pad.

A requirement to do this at a low cost in regards to a pipelauncher
is to accelerate the rocket at high gees.
There is no practical limitation on the amount force a pipelauncher can
be designed to deliver.
A pipelauncher is using buoyancy. A pipelauncher could said to be
a kind of boat, just as a submarine could be called a boat. Or you could
say the pipelauncher is a kind of balloon- but instead of air, the medium
it is using is water. So, the kind of limitation one would have, is comparable to
how much weight ships could transport- a similar "scale" of limitation
regarding pipelaunchers.
So a pipelauncher is a pipe with one end cap. If you take 2"
diameter aluminum pipe which is about 3' long with relativity thin wall, and
have a cap one end, then throw it into a swimming pool, you will see how
a pipelauncher "works".
This capped pipe, will let in some water, then it tip over and float
vertically.
That's what pipelauncher is. With the added complication of adding
or removing air inside the pipe in order to make it go up or down.

Generally speaking, for a pipelauncher to work it needs to be made
of material heavier than water. The heavier material functions in a similar
way that a keel of sailing boat prevents the wind from tipping over a sailing
boat. So if you make the pipelauncher out of lighter than water
[such as plastics], you would need to have some kind of ballast at the bottom
end.

The dimension of a pipelauncher could be about the same as a pencil times
1000- in diameter and length. Though some pipelaucher could be similar to
a pencil which is 20" long.
A pencil is 7 1/2" by 1/4" in diameter.
Times a 1000 is 7500" by 250" [625' by 20.8'].

As the title suggest I want 6 gees for 2 seconds.
So Distance = Acceleration 1/2 x Time squared

Gee= 9.8 m/s/s. 6 gees equal 58.6 m/s/s
So, 29.4 times 4 equals 117.6 meter of distance or length.
[3.2808399' to meter, so pipelauncher has to be *at least*
386' long.]
Now, I want the pipelauncher to start accelerating at 6 gees
do this for 2 seconds, then have gees [acceleration not velocity]
reduce until 0 over next second or two.
The velocity from 2 seconds at 6 gees is 58.6 times 2.
Which is 117.2 m/s [422 kph or 262 mph].
So in 2 seconds without any additional acceleration, the
pipelauncher will travel an additional 234.4 meters.
If the acceleration goes from 6 to 0 gees in such a
way that we get average of 2 gees for 2 seconds:
The added length is 39.2 meters. So:
117.6 + 234.4 + 39.2 = 391.2 meters
Or a total length of 1284'
And a total velocity of 156.4 m/s [563 kph or 349.6 mph].

As far as designing this you would design a pipe which
withstand the 6 gees for about 400', then tack on to this the
additional 800 or so feet of pipe length- it should fairly easy to
do. One could probably use modestly high strength steel or
marine aluminum.

One could design the pipelaucher to launch any size rocket, though
it would need to be a rocket that can withstand 6 gees of acceleration
plus the one gee of Earth's gravity. Humans of course could fairly easily
withstand 7 gees for 2 seconds.