Brian, what thickness of aluminum are you using for your battery holders? I may try your technique with some 1/8" thick polycarbonate, and then use heat (torch) to soften the plastic enough to be able to bend the tabs.
SH Z-Car, Custom Crawler, 8s Savage, 12s XTM XLB 1/7 buggy, 4wd 4-link rear/IFS Pro4 truck, Custom Hyper 10 Short Course, Belt-Drive Mammoth ST 1/8 truggy, 4s 17.5 MM Pro HPI Blitz