Well, I suppose you could mock up a cello fingerboard on a yardstick or the like. You could mark off the distance between fingers, and practice getting used to the distances. You can buy a decal that's meant to be applied to the fingerboard, and stick it on your proxy.
As long as we're on crazy ideas: You could see if a music store has a neck from an irreparably damaged cello and practice on that.
But I must caution, along with the commenters, against doing this. The common theme among the comments is: the main thing to learn is not just which finger to use, but where to put it, down to the millimeter. On fretted instruments, you can place your finger a mm higher or lower and it makes no difference, the fret stops the string; on fretless instruments it would change the pitch. This is not something you teach your cognitive brain ("B = 1, C# = 3"), but your so-called "muscle memory" (really a different part of your brain), and it's hard to learn and much much harder to un-learn. Silently fingering on a board or yardstick, even one with finger positions marked precisely, would put your thumb too close or far away from your other fingers, giving you the wrong shape for your hand overall. Even silently practicing on a broken cello neck would feel different if there are no strings.
It seems your motivation is to make the best use of your time: for some reason you don't have access to a real cello yet, but want to learn something in such a way that it will save time once you get one. Practicing with your hand in the wrong shape will cost you time rather than save it. If at all possible, get your hands on a real cello. A laminate beginner model can be had for about $800 USD (don't waste time and money on anything you find cheaper), or you could of course rent an instrument for a few months.
And if you want to make the best use of your time, get a teacher. I highly doubt that your bowing is in fact "passable." The bow hand, wrist, forearm, and upper arm move and interact in complex ways; I've seen some teaching systems that spend 6 months just training you to hold and move a dowel rod, while making minute adjustments to the position of each finger, motion of the wrist, etc., before giving you the real bow. I would encourage you to start your learning by plucking the strings ("pizzicato") and learn these bow skills gradually. Again, these are hard enough to learn correctly, very hard to know what you're doing wrong without a teacher's help, and very hard to unlearn.