The problem you will have with this is that this material isn't designed to be a spacer. It will quickly become saturated with fuel and exhaust residue and will either blow out and/or start leaking. You are going to come up with more problems than you are going to solve.

If this is still on the Nexstar you are building you really need to stop and figure out what is wrong with this plane. The Nexstars are a very well designed plane and if this engine doesn't fit correctly something is definitely wrong and should be addressed correctly, instead of trying to "rig it up".

Ken