There are speaker grill manufacturers who will produce a grill to your dimensions - eg www.speakergrills.co.uk
Before going down that road, you need to consider how you are going to fix it to the cab, whether you want a flat grill, or one with folded over edges etc. I would also recommend having it coated before despatch. You will waste a lot of paint spraying through the holes if you do it yourself.
The alternative, if it exists, is the aftermarket you refer to, which may be more expensive because each product is intended to fit an expensive branded cab.
David