Beautiful FH Theo!
what size is he now? His Kok seams huge for his size!
The bottom of your tank will be fine to drill. It will be no different drilling it now, then the day you got it. (sides and front/back panels are not though)
The bottom panel will be easier to drill now, then it was on day one. Acrylic absorbs a very small amount of water over time, making it more flexible. So drilling it will be easier.
The front and back panel will be a bit different as they will be bowed slightly. If they didnt bow at all, then they are good to go as well.
Drill it with the saw type of hole saw you would to drill into wood. If its your first acrylic tank your drilling, i have a video on how to do it.
I have drilled acrylic tanks that were set up for 2 years with out any issues. I've even drilled the brace on a couple tanks, that were still full of water.
All you need is the drill. And another piece of acrylic to build the weir. A router will easily make the teeth on it.
Total time to do both : less than 10 minutes.
Go for it Theo!