RE: Squish band
I just had a look at my old Taipan 2.5BB diesel and the contra piston has got a very definite squish band and a bowl shaped combustion chamber. It's also got a flat topped piston but uses two Schneurle ports and two exhausts.
With glows though and their single ignition point there's an optimum distance from the coil to the top of the piston at TDC. It's a ratio of the distance to the bore size and IIRC it's something like .15:1 or thereabouts. It's to do with getting the flamefront to spread out evenly and as quickly as possible so a fairly high combustion chamber is needed. A hemi is a good shape (although the Cox trumpet seems to be even better, at least with Cox's) and an added advantage is that to get a good shape and high enough compression it leaves you with a nice sized squish band.
The worst combustion chamber I've seen on a Schneurle engine was in my son's old SC 1.08 because it had a quite narrow squish band which quickly stepped up to a totally flat combustion chamber roof. We made a new hemi head with a much wider squish band and this gave an extra 500 revs. It almost certainly wasn't ideal but as a first attempt I was quite pleased with it.