Oh gosh. All is not lost yet. What kind of glue did you use? Is it water based or the old stuff or something like rubber cement? If you put enough glue in there, take the sheet, put it between two sheets of something like what new rubber comes in or between two sheets of plastic and put it on a flat surface and put something heavy like a stack of books on top of it for a day or two or three or four. See if that gets the rubber to stick to the sponge. If there is enough glue in there, it should, if you think you need to put more glue in, do that first. Don't put too much glue though.
It could be that the glue has not dried fully yet. If that is the case it is hard for the sponge and the pimples to adhere. When the glue is dry if the rubber is pressed from both sides the pimples should adhere to the sponge. If you used a glue like rubber cement or the old type of glue, that would expand the topsheet for while until it has fully dried and the VOCs are gone. When the solvents in the glue are fully evaporated the two surfaces should adhere. If you used water based glue that should not happen and then it may still just be a case of waiting for the glue to dry and keeping the topsheet firmly pressed to the sponge.
You asked if I have done this, I have watched it being done but I have not done it myself. I am friends with this guy who fixes everyone's equipment at the club I play at.
It is too bad it is too late to take the sponge back and see if they will exchange it though. As soon as I saw Wiwa's comment about taking the sheet back, I realized that was the best first thing to try. If you had not used the rubber two many times and it could be seen to be fairly new and in good condition, the shop should have taken it back regardless of when you bought it. I believe they can send things back to the distributer when something like that happens and get it replaced. If the shop did not take it back, you would have lost nothing but a trip, and then trying this technique.
But now the issue would be, to see if you can get the pimples and the sponge to stay glued together before you buy another sheet.