I obtained the William Optics Red Dot Finder Kit with Vixen Style Mounting Base, but could not attach it to my Takahashi FC-100DZ because the bracket supplied did not have the correct sized slots for the screws. I bought two other brackets: one was too long such that it interfered with the screw that locks the focuser in place; the other was too wide such that it did not sit flush on the finder mounting area.

The Farpoint Takahashi GSO Bracket Adapter fit perfectly, with screws that were short enough so that they did not protrude into the focuser tube when tightened, despite being longer than the ones that came with the OTA.