Get a piece of Delrin, not cheap, but it works and you can turn it with a wood lathe. For 1/2" toneboard, I would go with 3/4", drill all the way through with 1/2" bit, cut 2 grooves for O rings on one end, glue groves on the other. You will have to change your exhaust end and step drill it part way for the gutsert to seat in it. Your barrel, mouth piece should remain the same.
I would recommend testing this using scrap wood when you get started, it will save you some money. Turn a 1.75" long piece to 3/4" diameter and test run it a few times and then using it as a model for your final set up. There are no rules on how to make a gutsert, design it to fit your set up with what you have. You do not have to glue it in the exhaust end, you can use O rings. You have been good at thinking outside of the box, this should be easy for you.
There is some schedule 80 PVC pipe that can be used, but I can not remember the size I used, you can turn it on the wood lathe. Gray in color, it has thick enough walls for O ring grooves, but is not readily available everywhere.
Marvin