I believe they are usually made from tool steel and are bored out before hardening and tempering. I think the backside of the holes are slightly reamed out. I bet a usable dowel plate can be made without any of these refinements. Let us know how it works.
I have a 1/2 dozen i needed a dowel and drilled a hole in the back of a bar clamp hasnt hurt the clamp and the dowels worked just fine. I suppose if you were making 500 of them a tempered one would be nice but tying up most of a 50 dollar bill for a sometimes tool is tough to justify.
I guess mostly I was just saying that any piece of steel with a hole drilled in it will work if you put too many to close it may bend when you drive the dowel through the hole. The only thing to be aware of