If you're inner and out rings are fastened by the same bolts and as mentioned by others already I also think using a plastic ring, which has to support enough torque to keep the inner ring stable, is going to risk cracking. You could always try it and keep an eye out for cracks and check the tightness of the bolts but there are enough cheap metal types available on Ebay or elsewhere to make it worth buying one and as also mentioned already you could just keep the outer ring as is or file down the teeth for the same effect. Examples here. I just used a search for 110 BCD rings but use whatever fits your cranks if different.
bicycle 110 bcd chainring bashguards - Google Shopping