Maybe turn a dog-ear P-90 cover into a mounting ring for the mini-humbucker? Cut a rectangular hole the size of the pickup in the top of the cover, drill the pickup mounting holes in the cover, and suspend the pickup with springs or tubing. Then mount the assembly to the guitar using the dog-ear screws. I've never done that, but it seems like it might work pretty well. You might have to rout the body slot a little deeper, though.
Kent Armstrong may have a newly re-released HB with a center single row of screws.
Sweet SD solution, but they are proud of that solution. Avoid the Alnico 5 magnet in their minis, lack character.
I'd rather drill a new off-set row of screw holes in your covers and mount the pup inside. I did this by using cloth(topside) and foam(underside) of exposed coils. Simply sandwiched between cover and base. You could use all purpose silicone for placement and dampening.
Rio Grande offers exposed coil mini-hb set. A 1st.