Bleed Value problem - not vent all air

[SpaceMonkey]

I have a problem with a new radiator (badly designed) where I need a bleed value that bleeds from the top instead of the middle of the value. Don't know if there is such a thing, can't find one.

The issue is that the radiator has 4 upright columns that are linked at the top by a round pipe. The water flows up the right 2 columns through the round pipe and then down the left 2 columns. The holes between the columns and the round pipe are half way down the round pipe. So if I use a standard bleed valve that vents from the centre I am unable to bleed all the air out of the round pipe. As a consequence when the radiator is running I get a constant loud bubbling noise as the air passes between the round pipe and the columns.

I cobbled together a set of plumbing fittings that I screwed into the screw where the bleed valve goes which allows me to bleed more air out of the round tube. As a result I no longer get the constant radiator noise.

However, I really need a bleed valve that does not vent from the middle of the valve but towards the top edge. Is there such a thing or am I going to have to make a bespoke bleed valve?