If you are willing to spend a little bit of cash experimenting on this I have an idea.

Most builds tend to be out of aluminium (weak) or steel (need welding skills) even though it has been known for a rather long time that big lumps of cast iron or granite do the job better and modern epoxy granite is much better still.

If you want to carve out a niche instead of offering a full cnc machine offer a standard gantry design that makes a brick shithouse look like it's a straw bale, but comes in user defined lengths.

You will probably be able to put food on the table doing this because buying small quantities of epoxy costs an insane amount, if you can invest a few grand up front to buy the epoxy can get a lot more for your money. By contrast the bulk cost of steel/alu vs the retail is not that great.

Make sure that you can mount either supported round rail or hiwins, have a seperate foot plate so that people can choose their own block spacing and you have pushed the hobby market up a notch.

PS: If you are after some cheap but rusty steel up your way I found these guys the other day:

Steel Beams and Columns, Gates Fencing and Railings items in AinscoughMetals store on eBay!