Do you know what drivers were planned for these cabinets?
If not you need to measure the internal cabinet volume and the (small looking) port cross-sectional area/depth. Then check for any bracing or lining/stuffing. It'd be super cool if the back panels can be removed.
In recent days I've been ranting against MTM design for audio, due to the need to lock your head into the midpoint plane of the drivers to avoid phase problems. But perhaps you can come up with a MTW configuration (midrange on top, tweeter in the middle, and woofer on the bottom).
A common mistake is to ignore port location in tower speakers. Drivers up high with port near bottom like yours is typical but in effect creates a 'pipe' (more specifically a transmission line) versus a simple ported box, so the sound will not be 'right'. Transmission lines (TL) can be designed but it's much more complicated than ported boxes. You might try contacting Bob Brines (brinesacoustics.com), he is well versed in TL. Again with the MTM design, it can't work correctly for both drivers (the geometry can only be correct for one of the drivers.
Frankly it's doubtful that a viable TL can be made out of these cabinets (especially with the narrowing), so your best bet is to plug the port and make a new one of the back say a 1/3rd or half way up based on port design (simple software can be found on the web). If needed for the chosen drivers the bottom can then be filled with sand/shot to reduce effective cabinet volume.
Take care.
