It looks like compression will hold them in pretty well, but I'd use a zip tie in the center to hold it in place against the plastic vanes. One in the center or two offset to the sides wouldn't look bad or be difficult (or expensive) and would make the block a part of the vehicle instead of just something jammed into a hole.
Nice job- how are you liking the warmup time now?
Edit: maybe "weaving" it into place by passing it behind one of the vanes would work. It'd probably be way more work and look less smooth than a zip tie, though.