I did this a couple times and it works well enough. Your correct, the idea is to get clearance between the barrel and forend and with the rubber spacer, resting the forend could cause contact.
Personally, I use the "O" ring and neoprene washers as tests. if it works, I either pillarbed or RTV bed the forends. I have only done 1 RTV. it worked very well but I'm not sure it will work all the time. IMHO, pillar bedding is a better more fineate solution.
CW