There's a link to a guy does custom acrylic reservoirs etc
In one of the sub forums
Would assume he can do a side panel too
Since it's still CNC acrylic
Though it might not be cheap
Will see if I can remember where it is
There's also a couple of forum members have CNC machine
But the names totally slipped my memory
Here's the link
But still can't remember the members with CNC machine
You could try post asking if someone has one
Perhaps in the wanted section if you have access to MM
http://www.productdesignfoundry.com/
Edit here's a sample of the work
They did for forum member
https://imgur.com/gallery/Brd994O
Edit 2
That cost £250 including next day delivery
But obviously it's way thicker acrylic than a side panel
And more complicated to make
That price is pretty much standard for that sort of reservoir
If you look at work by radikult