when using a topless form, and rubber bands, for making boo backed bows, i took a strip of the hard insulation styrofoam, cut it the width of the form and about an inch thick.
i took a piece of wooden broom handle- wrapped some 60 griot around it, and quickly sanded a depression in the surface to match the boo.
lay up the bow, some saran wrap, then the styrofoam, then my backing strip, (which was a wood strip, slightly trapezoid shape- in order to spread the load right across the width of the bow evenly- i guess a half round piece of housing trim 1.5" wide would also do well).
then the rubber bands.
the foam would pull down nice and evenly onto the boo- and get crushed/ depressed in perfectly at the nodes- and would apply great, even pressure over the entire boo surface.
the foam was not reusable- but a piece of foam goes a really long way!!