Use the pixel chassis in FOTEPX's DSL CE, along with ra2cf. Build your bot's drive, then weapon motors, then batteries, then build bodywork over that. This order of building is the easiest way to make decent extender it's
Also bottom panels are a complete waste of weight unless you're building a pure artbot. IRL bots are already weak as hell in combat, you need every kg you can get put into drive/weapon motors so your bot doesn't get slowly tapped apart over 3 excruciating minutes of nothing. Of course, if you're not going to enter your bot in tournaments or you want to build artbots this doesn't apply.