Since I'm in the process of making pretty much the same thing, this is my approach. I found a discarded piece of acrylic near a dumpster and promptly liberated it. It's been sitting in my workshop for about a year and now has finally found its calling. It is a bit scratched, but beggars something something choosers...
I cut it to size on table saw and cut out a rough hole on scroll saw. Then on a milling machine bored out the hole to size, which made it nice and round with a clean cut. The hole is 140 mm (5 1/2'') in diameter.
Then on lathe I turned two aluminium pieces between which I'll sandwich a mesh for ventilation. The two pieces are held together by some screws (I'll probably use countersunk screws for final version).
The aluminium piece can be removed for maintenance and feeding. There is a bit of room around the ring for fluon application.
Next on the to do list is to cut out a square a hole for a thermometer / hygrometer.
This is what it currently looks like:
I would have done it on laser, but I was a cheapskate and bough the smallest thing available, which has a working area of 20 x 30 cm (8 x 12''), while the lid is 20 x 40 cm (8x16'') and the acrylic doesn't physically fit in the machine.
On laser it would have been a 10 minute job, but now I'm spending hours doing it. I don't even know what I'm going to do about the square hole, probably mill most of it out and hand file the corners.
I could have somebody else do it for, probably, fraction of the cost of my time, but who charges time on own projects, right? It's good fun really.