First try quarters, some token mechs take tokens and quarters.
If not, replace the cradle on the mech to a quarter one.
Or get quarter mechs, like these http://na.suzohapp.com/gaming/coin_comp/35000300.htm
I like the metal ones. And note the posts on the sides, there are three holes, you can unscrew and move them, or remove them, depending on what style door you have.
LTG