For HT, the center does most of the work and you will not be happy with anything that fits in the 5"x19" cubby you have there (Nor is a cubby an ideal location for any speaker). Best solution would be to wall mount your tv and put a proper center under there. Also, if possible, spread out your L/R more to open the front soundstage. At a minimum, pull them forward to be in line with the front of the tv stand.
As far as short centers go, the shortest that comes to mind to me is the Polk s35 at 4.1"...but it's 24" wide. Might fit on top of the tv cabinet and still be under the tv?
Marantz SR7013, Polk Signature S60s/S30/S15s/HTS12s, Polk MC80 (x4), AC Infinity T9/S9, AAB4 HTPC, Panasonic DP-UB420K...still to come - 82"-85" 4K QLED or eqv.