I have limited experience, but when I glued my cushions last week, the contact cement was pliable enough to manipulate positioning without much problem.
I used Barge(tm) cement for most of them, with a 10 minute setting time before putting the cushions on. I then ran out of Barge, and had to do the final rail using a locktite adhesive (unable to find any more Barge). The locktite instructions called for a 5 minute set time, but this did not have enough grab, so I did more like 7 minutes. The locktite was also slower to cure, but once it did, it seem quite firm. With either one, you have some "wiggle room" to get them set where you want them.
The best way to put the cushions on is to work left to right, using left hand to position cushion on rail, while right hand holds the rest of the cushion up and away from the rail. I put my rails in a workmate vice while putting the cushions on. After about 15-20 minutes moved tham to a table for 24 hours dry time.
If you use Barge, get three tubes.