I see you already painted, I hope you didn't paint around the axles holes, if you did carefully sand it away with a small piece of sand paper, just enough so the wheel hub doesn't ride on paint. Also take your time to get the axles straight into the slots. If you have the tool to drill the weight holes that will help, although I am not sure how it will fit on your curved back end. When removing wood to reduce weight I would start with a smaller drill bit and work your way up to a bigger drill bit. And as suggested by Duckofalltrades marking the drill bit for depth is a good idea and stay away from the sides and axle holes, it's easy to take a big chuck out or splinter the wood if not being careful. Good luck.