I use a male to male nipple through the keg wall and put brass and fibre washers either side to seal it that are clamped in place by the tap on the outside and whatever fitting I use on the inside. The washers can be found easily at automotive shops or hardware shops and need to be made slightly larger to fit over your 1/2" or 3/8" fittings (or their metric equivalents"
I can source/ sell 1/2 inch SS BSP ball valves for 25.00
Use a 1/2inch male hex nipple these are around 8.00ea
I use fibre and copper washers on the inside and a rubber and copper washer on the outside.
5.00 for 1/2 inch ss lock nut.
for 12.50 ea i have 1/2 inch SS nut and tail set up that you can clamp your ss braid onto.
and for 15.00 ea i have 1/2 inch ss tpiece taht you can use for clamping the ss barid onto to make a ring.
I got my bits and pieces from Tony Powell. I use a compression fitting on the inside (male) to the female side of a ball valve, then onto the male of the ball valve have a T piece and a barbed fitting. Will take a photo tonight if I remember.
Call Tony Powell and Prochem to get some prices.