I also have a modified bumper jack. We used quarter inch nylon rope through the bungee and cut the loop after the bungee was installed cause we weren't ever gonna get that knot untied. One loop broke on our very first attempt. Two loops worked for first and second bungees. When two loops broke on the third bungee my fat ass ended up in the rear seat and I was desperately clinging to the jack with both hands cause I thought either it had gone through the instrument panel or I was now dead from the explosion. Amazingly scary how much force it took to get the third bungee installed. Installing PA12 bungees is serious business and you could get hurt or things could get tore up.