I did cobble something together using a pencil eraser, and 2 long velcro straps, but it wasnt ideal

I really like the look of those Exposure helmet Mount Brackets:

My helmet seems to have diagonal vents, which makes the lockblocks really difficult to fit and gte it pointing in teh required direction.

 Just reckoned maybe somebody somewhere had got the exposure mount to work or even had found an alternative, cheaper supply

I know one person who uses the fenix 360 mount (on that page linked above) but it's a bit clumpy.

I'd have thought your helmet has at least one straight front-to-back section?  The 'in-line' lockblock will sit on that just fine & you can get it lined up pretty well before  tightening.

I use  a lock block for mine with no issues.

There's a chap I know who fashioned his own mount with plastic and zip ties that is very effective. You can buy the plastic in Maplin in pellet format - add to hot water then mould into shape.

Polymorph

The TK11 has a 25mm dia body to match gun rail mounts, just a bit too big for the Joystick mount, at about 23mm.  It's extreme, but another option could be to file or machine part of the body to reduce its diameter, as it is quite thick walled.