Glad you find a solution, Felt clunks work very well, and some people even use them in the main tanks without a header tank or a uat with success.

The original problem with your header tank is that it is quite small, and its standing in the up position, with a small cross section area. I always used bigger ones, with at least 10oz and the tank shape itself as close as a cube as I can find, with the biggest cross section area in the horizontal position.

The only disadvantage of the felt clunk is that it needs inspection every flying season or so, and it increases fhe workload of the fuel pump. This is not a issue at all for the pump mechanics by itself, but for helping avoiding trigger that dreaded watchdog function some, if not most, turbines have.