I have now found a brass connection that can be used as support for drain-valve. Head is tapered and size reduced to fit better inside the plastic tank. I cut a thread1/8 NPT inside for the drain-valve. Outside is 1/4 cylindrical pipe thread, with a nut downside tank. Installed a fuel-return connection on inner side tank, a ventilation connector at fill-cap and the drain-valve at lowest point.
