Don't know of a metric supplier, but BA roundhead screws with no slot are available from many suppliers.
(Yea, yea, I know BA threads are metric, but you know what I mean).
EKP Supplies do 'em.
Livesteammodels.co.uk also do threaded rivets with slightly longer shanks than those available from EKP – again in BA sizes.
Alternatively you could turn your own slotless roundhead with a form tool on a standard 3mm metric hex-head, or simply fill the slot on a standard round head after assembly with filler or thick paint.
5BA & 6BA have top diameters of 3.2 & 2.8 mm respectively.
Regards,
Phil
Edited By Weary on 14/06/2014 22:41:44