What size of Surge tank for ET2

PVC pipe will work if you use PVC cement, not epoxy and you pre regulate down (as you should if your pump already has a tank) before the surge tank. I am not sure of the burst pressure of PVC, but your issue was with using epoxy. PVC cement is a "weld" of sorts, good for at least 40 psi.

Two 2 liter tanks with a small (1/4") hose or additional regulator (making it RCLC ?) between should do fine. If you chose the above suggestion to put batting in the tube, make sure that you have a very good filter on the output and allow it to pump air directly out for a bit to clear the system. Something like the ones used for plasma cutting were there is a internal replaceable filter that looks like a toilet tissue roll. Make sure that you adjust the tank pressure down to about 3-5x final pressure. The pump will work quieter and more effectively with less water condensation.

What are your starting and ending pressures? Original manifold or the high pressure manifold?