Hey Gill,
Congrats on the Helix 5. I should mention, I have no affiliation with Raymarine, so take my info for what it is, hopefully it helps someone.
The Dragonfly ice kit does not come with a mount, but has a base that your existing factory mount attaches to.
That is how they have made it compatible with the newer and older models. It is give and take I suppose, you have to move the entire mounting base and bracket not just the head unit. I attached a couple of pics
The Gen1 Dragonfly 7 uses a similar system to what you describe, setting into a cradle that clips on both sides.
I would love to get my hands on a spare Legacy Dragonfly 7 mount so I wouldn't have to bolt/ unbolt every change of season. I managed to get the part number ( R70302 ) from Raymarine's tech support, but the mount is discontinued and I cannot find anyone who has them in stock.
We will see how it performs on the ice, unfortunately the package arrived about 2 hours after I left for Friday's fishing trip.