I see two ways of mounting miniature DPAs to these balls. The first and easiest would be to simply gaff tape the mic to the face of the ball so that the mic grid is held against the surface of the ball and the mic body is tangential to the surface. I'd point the grid upwards if orienting the ball/mic combination directly forward or angled out +/-45 degrees. Angled +/-45 degrees is how I run mine and what I'd suggest for folks using them as a spaced omni stereo pair alone at decent spreads. If using the balls to help make the most of a situation where you are not able to space the mics as far apart as you might like (even though the telescoping bar allows for wider spacings) or if the intent is to use them in combination with a center mic or center pair, I'd suggest taping them to the outermost side of the ball (+/-90 degrees or 180 degrees opposed to each other). Sort of a double mini-jecklin / dummy-head setup. That will maximize the level differences. In that orientation, this taped-on tangential mounting would then allow one to point the mic itself upwards or forwards by rotating the ball. That probably won't make much difference, but may have some audible effect at the very highest frequencies.
The other more typical way of using the balls, and the method which would need to be used for any mic larger than the miniature DPAs is to make a hole through the ball for flush-mounting. Rather than drilling, which probably won't work well through the foam, I'd suggest either punch-cutting a hole through the ball with an edge-sharpened stiff metal or plastic tube, or melting a hole through it with a torch-heated rod or nail. A red-hot nail head and a steady hand worked nicely for making holes in my Nerf hard-foam balls. If the foam ball is compliant enough you might make the hole slightly smaller than the mic body so that it fits snuggly with no gap at the working end. A section of plastic drinking-straw just larger enough to pass the microphone through, running most of the way through the ball but stopping just short of the surface where the mic peaks out will make it easier to thread the mic through. If the ball surface gripping the mic body isn't enough to keep the mic in position, a short stick, tapered along its length (I use a piece of chopstick) inserted into the back end of the ball will wedge the mic cable in place without stress or damage.
To achieve variable angles, you can either make multiple balls and switch between them, or poke multiple holes through the ball through which the mic is inserted, or have multiple mounting points for the telescoping arm attachment around the ball. Mine are setup with a single hole for the microphone and two different blind mounting holes which slip over the antenna button end with a simple interference friction fit. That gives me options of pointing them directly forward (or behind) or +/- 45 degrees. I should make another set of holes to orient closer to +/- 90 degrees parlty to accommodate overly narrow spacings, and to see if that offers any further improvement in combination with my center mics even at wide spacings.
The important part is to not have an opening larger than the mic body unless you stuff something in there to fill the gap, and not have the mic stick out or be recessed in the ball. Ideally only the grid of the microphone should protrude past the surface of the ball, and the the ball's surface should align with the edge of the solid cuff portion of the grid. It may seem that I'm contradicting myself in suggesting simply taping the mic to the ball, then getting overly particular about the fit in a through-hole, but gaps or small pockets produce more of an audible effect than the mic sticking out slightly. Consider the difference in response produced by the long high-boost grid verses the short low-boost grid. Thats a similar acoustic EQ difference produced by a difference of only a couple millimeters of air volume between the mic element and the grid opening. Recessing the mic in the ball, or a gap around the mic is in some ways similar to changing to a longer grid.