Center channel speaker to match Snell Type J speakers?

IMO... just go buy them (ideally two used pairs) that you can find used on hifi shark for easy budget money

https://www.hifishark.com/search?q=snell

NOW....

(1) Mothball / pack up one speaker aside from pair #1 as a future repair backup, and
(2) Hook up the "other" speaker from pair#1 as your new matched centre channel to your LF and RF main set, and
(3) hook up the matched speaker pair#2 for your RR and LR surrounds;

and voila! .. you have a matched 5.1 speaker set sonically and appearance-wise as requested .

Option 2....(arguably preferred in the long-run)

depending primarily on
(I) your budget and
(ii) speaker shipping logistics, AND
(iii) assuming that you are still enthusiastically married to the SNELL family of speakers; and you will be able to find an antidote for any contagious "upgrade- itis" affliction going forward, 


THEN here is what I would do....

(1) Upgrade your existing main SNELL speakers (RF and LF) to the best model SNELL available , assuming there are other upgraded models to fill that niche. If so, then this move is an upgrade for your 2-channel audio.
(2) reposition your existing front speakers to serve as the "new" RR and LR rears in a 5.1 setup.
(3) Purchase one used SNELL speaker pair -- matched as close as possible to your rears in (2) -- available in the used market as highlighted in the prior post. This will provide your CC and a mothballed spare.

Every brand of audio equipment , including speakers, has its own sonic signature .

Within the available used SNELL model variants -- especially for a 5.1 setup  wherein the CC and RL and RR are a non-critical  manufactured audio track--  , it won't matter to have that identical variant to your main fronts. To identically match them all is a nice-to-have, but not a must need -to-have choice. 

its just more if a speaker appearance matching thing .....  speaker (veneer colour and finish .... IF that is even a hyper-critical issue at all for your 5.1 speaker expansion plan.

in any case, even a slightly different newer/ older variant  of the same SNELL model speaker to serve as your CC, will be a quantum leap closer (if not even identical) then any 3rd party speaker choice, to matching the bespoke sonic signature of your current fronts and blend seamlessly with the fronts , .... full stop.

because the rears in a new 5.1 rig  are just a pure fill-in manufactured sound track , it won't  matter by comparison to try a clone to fronts. 

IMO just go snag a couple of used pairs to flesh out your multi-channel speaker expansion plan.