I have spent ages looking back through the archives but most info is related to high back boosters.
We have a 5 yo and a 1 yo. The 5yo is in a high back booster and the 1yo is in a MaxiCosi Priori. So the plan is that when DD gets to 33lb (she's been 27 lb for ages so will be no time soon) she will go into DS's high back booster and DS will have a booster cushion.
However as DS gets older we increasingly find ourselves ferrying one or other of his friends around. So for convenience I was thinking of investing in a booster cushion that DS or a friend (whichever was bigger/heavier) could use when necessary now but would be used for DS once DD has his seat.
But the Which survey doesn't seem to cover the cushions. The Britax Horizon is £10.99 from Kiddicare and comes with a seat belt adjuster strap.