You dont need to sand the bottom of the chassis, just put duc-tape on the bottom. Flip the chassis and fill the slots, and let it dry overnight.
if you desire you can push in lead inserts into the epoxy while it is drying. I use the sheet lead they use to seal roofs. when cut to size there about 18g. per slot (see 3rd pic). I have 4 of those, and got a 40g plate under the ESC
btw, I like you paint. Looks just like mine
