None of the new 406Mhz ELTs use simple to replace D size batteries because they have more powerful transmitters that require more battery power.
The only option you have is to buy an 121.5 Mhz ACK ELT from from someone wanting to upgrade. The panel mounted control panel is pretty much teh same size as the ameriking.
Any opinions expressed in this message are my own and not necessarily those of my employer.
RV-6A (aka "Junkyard Special ")