This is a 3 post variation of Roukie68's drill guide using 8mm stainless steel rods. Fits my DeWalt DCD780 just fine and should work with a DCD777 among others as it uses the same dimensions as Roukie68's design.

Rod lengths are up to you. I used 3x 265mm rods but anything around that should be fine.

I used a 5/16" drill bit to drill out the base holes and a 21/64" bit to drill out the mount holes. Then I used a heat gun to warm up the ends of the rods and hammer them down into the base.

Fusion 360, SketchUp and STEP files are included.