I did a easy cheesy battery mod for my 3s and 4s lipo, I just wanted to share some picture of it with you. It started with a no-hardcase 3s and a too-big-to-fit-traxxas-battery-extension 4s lipo (you should check dimension before buying it.)
Sadly, I forgot to take pics during the process, so here's just the final product and how I did it.
1.5mm thick plexiglass
50mm and 60mm diameter transparent heatshrink tube
First, I prepared the 3s lipo battery by removing the heatshrink and disassembling the 4s lipo hard case (EXTREMELY DANGEROUS OPERATION HERE, don't do it if you aren't aware of the risk)
Mesure the dimension of the battery, cut the plexiglass and glue it. (not very much decription here, I know)
Heatshrink the heatshrink over everything.
Failed attempt at heatshrink, it ripped off brutally....
I will put some shoe goo later on this to waterproof everything
Molded to fit the hold down
Pretty fast mod , took me an afternoon to make. You might choose any other thin plastic material, I'm not sure if I did the right choice with the plexiglass, as it look to break easily. I saw a tutorial where the person used lexan.