Well, I had a chance to launch with the folks from MDARS yesterday (thanks to Kevin and the gang). I did several more launches of my 'zipperless piston' design before a had a failure that will require some rework...
I did several launches (3-4) on H-I motors with intentionally short delays. In every case, the design worked 'well', but in 1 case the piston went farther than intended because the tie wrap was a bit loose. I should add the for my first couple of launches (previous to this date) I had pulled the tie-wrap as tight as I could with a pair of pliers. I recently obtained a real 'tie wrap tool' from Fry's, and had the tension set a bit too loose, so the tie wrap ended up not as tight.
For my final launch with the design, I used an I435T, but the piston shattered at ejection due to the bending forces. The good news is that the body tube is undamaged, so I simply have to replace the coupler/bulkhead assembly, so (overall) I'm ahead of the game, but it means I still have some improvements to make. On the other hand, for 'nominal' flights this is probably a good design, as I doubt if I will be intentionally launching with short delays too often .
Somebody at MDARS suggested using the bulkhead plate with vertical 'spars', such that looking at the end of the assembly would be like looking at an 'X' or an '*'. This way, there is maximum area for the exhaust gases to disperse, plus the stress force is distributed a bit better than on a slotted coupler. I'm going to take a look and see how difficult that would be to do.
I'm a bit concerned about the smoothness of such a configuration as a 'piston', but I'll have to experiment to find out.
Any additional comments on this would be appreciated.