Modify the dimensions (RectangularHolder.scad) to fit whatever device you have. (Generate and print the small "test piece" to make sure you got the measurements right!).
Also measure whatever suction cups you are using and if they differ from the ones I used adjust and re-create the SCBase part.
I suggest that you use the SCBase_Strong part, as it is designed to be printed vertically so that the plastic layers are in the right direction to resist snapping off under pressure. The Regular SCBase part looks nicer, but it really isn't printed in the right orientation for strength. You can make it work if you are careful and don't overly stress it out, but the "Strong" version is better.
(I have used both 20mm "small suction cups" from a hobby store and larger suction cups taken from a sink scrubber holder from a dollar store...both suction cups fit into the same holes despite being different external sizes...) With some elbow grease you can generally smash and twist most suction cups into holes much smaller than it looks like they should be able to fit.
Print two Ball Holders (they go on either side of the ball), Two Ball Mounts (they fit back to back to make a single ball), one SCBase and one Rectangular holder.
For the Ball Holders, you can either count on your slicing software to add appropriate support material, or you can print the version with a round "break-away" brim already in the STL file AND use support material.
Once printed, drill out the SC Base hole using a 3mm (1/8th inch) drill. Use the same drill to drill out the six holes on the back of the rectangular holder (from the rear to push support material out of the countersunck screw head holes out).
Use a smaller 2.5mm or 3/32 drill to drill out the support material in the three holes in the base of the two ball mounts (6 holes total). You can also drill out the holes inside the ball, but probably don't have to.
Clean up the ball holders by breaking away any support material (including the designed in super thin ring on the "SUPPORT" STL file) and sanding or filiing away rough edges. Also file down any bumps on the ball part of the ball mounts. Hit them all with a heat gun if you get discolaration due to sanding/filing.
Use an M3x20 screw to hold the two halfs of the ball together with 2 washers and a nut. (It will self tap into the plastic, and a nut and 2nd washer is not truely needed, I used a M3x16 screw and didn't have a washer/nut on the other side.) M3 screws will self tap into these holes.
Use 6 M3x10 flat head (I used philips) screws to hold the rectangular holder onto the base of the ball mount. Be sure to run the screw around inside the rectangular mount a few times to make sure they all fit into the "countersink" hole far enough to not stick out of the plastic and scratch the back of your phone before screwing any of them into the plastic of the ball mount. Note that they will self tap into the plastic of the ball mount, so the nuts on the back are optional, but I used 6x black anodized M3 nuts to look cool. Use M3x12 screws if you want room for washers.
Using a twisting motion, "screw" your suction cups into the three holes on the SCBase.
Lay the two ball holders on either side of the ball and the Suction Cup Base (SCBase). N.B.: Make sure the rectangular holder is "up"! Use an M3x16 screw and two washers and a nut to attach it to the SCBase. (You will have to self tap the threads into the ball holders on either side.) Adjust the ball holder "arm" to the angle you want and tighten it down. (Or leave it just loose enough to move if you want to readjust it more later...)
Use an M3x25 screw with 2 washers and a nut in the 2nd hold, tighten it down until the ball can just barely be moved.