If you are familar with the sage industries EBR stock --- it's an aluminium stock / chasis for the M1A or Mini-14 --- that might be the ticket.
Unless you have an adaptor to mount the M203 on a pictinny rail you will have to either fabricate something or machine the EBR stock to take the 203 --- easier than other options since it's metal.
Why is a whole other question.
