Hi, John, and welcome.
The commercial plates are, indeed, expensive, all things considered. The only wrinkles with making your own is that you need to be able to mill a rabbet around the hole to accommodate various sized inserts, and be able to precisely align the mounting holes to attach your router. You'll want the hole to be large enough to accommodate the largest bit you'll use, but you'll want inserts to cover part of that hole for smaller bits.
Thickness of the plate is a trade-off between strength (lack of sag over time under the weight of the router) and diminishing the depth of cut possible. Some like the phenolic plates, others metal. Bare aluminum is not a good idea, since it will mark workpieces.