Injected EPS beads & wall plate cavity closer

For various reasons, 120mm cavity has been closed with a 150mm fire cavity sock at the wall plate. Soffits and fascia in place. (pictures)
Cavity insulation to be injected in (bonded EPS beads).

Challenge: how to keep the EPS beads from shooting out between firesocks, or between firesocks and lintels. At EPS bead installation time.

Looking from below, light gets through at firesock joints. And between firesocks and lintels. 

Builder thinks 2 upstands of celotex mechanically fixed to the inner leaf/wall plate exterior will suffice to close the gaps and keep the EPS beads from shooting out of the cavity at the top. But he has zero experience with EPS beads.

What to do? (Cost is a consideration)
Some offcuts of hollow PVC soffit are lying around, if they might be useful.

Could Tyvek Housewrap (airtight, vapour permeable) above the firesock be used? 

Edited February 15, 20224 yr by WWilts