So, in order to prevent a mould distorting under vacuum (especially when you make it from thin absorbent fibre-board) you need to stiffen it, generally by bonding it to a backboard. Previously I bonded the other mould to a backboard with bog. It worked well and made everything solid, but used a load of bog in the process (well over a kilo).
So, next approach was to try bonding with expanding foam, a technique which does work.
What to do – squirt in small amounts at regular intervals so it just makes contact whilst keeping pressure on the top so it doesn’t wobble around
What not to do – squirt a shed load on the backboard, lay the top on and leave it overnight.
This is what it looks like, all distorted and buckled, way beyond the application of bog to fix it. This is what it looks like just before it goes into the skip.