I just did an electric conversion on my seagull decathalon. I'm running an AXI 4130/20, Jeti 77 amp opto, on 6S. The plane flys great and balanced out great with 2 3700 MAH PQ 3S lipos mounted against the firewall. The ESC fits nicely in the original muffler box. I removed the back of the muffler box to promote airflow and opened up the oil cooler spot in the cowl for airflow. Here are some pics of the conversion. This plane has no right thrust built into the firewall, so I added a .062 thick washer under the left side stand offs to give me 2 degrees. The stand offs are 1.0 inch long and the plywood pieces are .250 thick. The reason for the plywood was because I didn't have 1.25 inch long stand offs and to block off the original motor mount holes.

Here are pics of the battery hatch. It is under the belly, behind the LG. This plane has a great spot to fab a hatch, it has a large area that is wide open and only covered with monocoat. The hatch is light ply and is fixed in place with a hook at the front and magnets at the back.

Here is a pic of the battery tray. It runs all the way up to the back of the firewall and hold 2 3700 MAH 3S lipos (650 grams). I hold the stacked lipos together with velcro.