Novak actually has made a humbucking/coil-split-able version of the Bisonic. I don't think he's added them to his website yet but I'm sure he'd be happy to make you one if you email him. They have the same look and physical dimensions of a standard Bisonic from overhead but may need the cavity to be enlarged a bit to accommodate the extra coil. Haven't heard it yet, but I'm sure it sounds great, going by his regular Bisonic options which I've logged plenty of time on and love.
With the Guildbucker I think you would have to make the cavity larger to accommodate the height adjustment screw tabs on either side. As others have noted, you'd either have to drill new mounting screw holes or make a new mounting ring to use the original ones.
Another direction to explore is one of the following (none of which have screw holes that line up perfectly to Bisonics):
-Dimarzio Model One pickup, which I am a big fan of for the big tone and their commendable flexibility (4-lead pickup for single/humbucker/series/parallel/phase wiring and switching options). I like them best in the neck position, but they do work in the middle and bridge positions as well. Originally manufactured as a less-muddy, more flexible drop-in alternative to the Gibson Mudbucker. Should normally drop right into a Starfire cavity, though they are encased in a big ol glob of epoxy which can vary a bit in girth and shape.
-Schaller and Dimarzio bass humbuckers in the regular guitar-humbucker-sized format. Both of these are practically identical in their specs and appearance and do have the correct string spacing. I like them a lot too. Similar tone to the Model One humbucker (maybe a little more mid presence and slightly less low end) and they have all the same 4-lead wiring options. Have height adjustment screws on either side which MIGHT require some widening of the pickup cavity.
-The rare Shadow humbucker that has the correct spacing and is most similar in tone to the Shaller/Dimarzio guitar-sized ones, though the physical foot-print is closer to the Dimarzio Model One. Also has height adjustment screws which will likely require a widened cavity.
-Vintage style Thunderbird pickups like the Novak offering are often surface-mountable humbuckers that you wouldn't need to use any cavity for. Curtis recently showed off a Mustang bass on his social media that had one in the bridge position and supposedly required no routing. Look really great in all chrome, with no visible pole pieces.
A few more (though I have had no experience with them):
You could also explore the Gretsch and Rickenbacker style humbucker options out there. I would expect them all to be smaller than a Bisonic so a custom trim ring or mounting ring may have to be fashioned for them.
Bartolini made a Guild-size humbucker that I believe was designed to replace Guildbuckers. There has been some discussion here on the board about them. They also make Rickenbacker sized options.
Then there are also a few guitar-humbucker sized bass humbucker options from EMG.