Most soft domes don't require much suction to fix when pushed in, so the cardboard tube idea should work. If not, do what I have done with my Dynaudio tweeters: use your mouth. Not kidding. Just place your lips very gently over the middle area of the tweeter and suck gently. Just make sure to not drool. ;-)
Can a "pushed in" dome tweeter + midrange be savev
Grilleless Gershman Sonograms were approached by 4-year old. All six speakers were "pushed in". I'm wondering if there is any way to create a suction or something that would return the domes to their original shape. There is a soft dome tweeter but also a Morel domed midrange. The dust caps in the woofers are pushed in somewhat but I don't think it matters.