In reply to Milesy:
I'm using the petzl trigrests, they do fit old quarks nicely and are a good shape, but I have broken one already and I know several other people that have too. I'm hypothesising this is exhaborated by not having the trig part in there as there is nothing solid between the plastic parts that screw together. I've now packed the space with washers in the hope they will be stronger but I still think they are on the flimsey side. They break when they hit rock/ice. Personally I carry spares for multi day trips to replace them with if they break.
The Grivel trig rests I think are tougher. I bought some last year but I haven't got around to sawing off the trig part and fitting them yet as I don't have a metal saw and/or vice.