If you have access to tools, you might consider cutting a hole slightly bigger than the dented area in a bracing material, perhaps 1/4" plywood would work. If the dent is say 1', cut the hole about 1 1/2" and bevel the edge that will go against the cone so no sharp edge will damage the speaker. This would be held against the perimeter of damaged area while you try to pull the dent. Not knowing the textural characteristics of the area that is dented, your options might be; if the area is smooth, an appropriately sized suction cup might work nicely. If textured, an implement tipped with a ring of something like blu-tack might allow adequate suction to pull out the dent. It all depends on how sturdy the dented area is, the material, and if you can devise some means to get a pull on it. Perhaps a call to the speaker manufacturer would be in order for suggestions. Good luck!