This is easy. Castincraft clear resin. One or two 12 dollar cans should do nicely. This stuff is designed to submerge stuff into. When submerging they recommend you do it in layers, and so do I. Especially for something that size. For the air bubbles if you use this stuff, for one stick it on something that vibrates good. Also you can use like a hatpin or something to get in there ti help draw them out. This stuff also has a long cure time. Takes a good 12 hours before it starts to really set up, so there is plenty of time for the air bubbles to rise out. Since I doubt it would take you a full two cans, it might be worth considering picking up like a VERY small diecast. Like hotwheels sized armor. Then try submerging that. GEt something that is about as complicated as the 35 scale tank. That way you can judge how the airbubbles are gonna go, and where they would be at. Gives you some practice. The only issue with this resin(other then it stinking like bondo), is that any air will cause it not to cure and the surface to be tacky. There are two ways to fix this, one is an additive, which you can order from tap plastics. The other is a spray that after it's done you spray several coats on, and it helps to cure the surface. It is a clear coat, and after the coats are dry, you can polish right up. It is made by castin craft as well. For the 12 dollar can the stock number is 00175. You can get this stuff through tap as well. I use Tap's own quickcast for casting regular resin and their silicon rtv because it's every strong, very flexable, and most of the time you don't need a mold release agent. Tap also sells a clear casting epoxy as well, that is very clear and has a good surface and polishes up quite nicely.
*EDIT*
Reading some of the other posts reminds me. The castin craft doesn't give off the heat, since it's such a slow cure. Neither does the casting epoxy sold by Tap. Jes give em a call, and visit their website. They REALLY know their stuff. If there is a location near you, take the model in, and the base, and tell them what you want to do, they'll get you set right up. By the way, this is an absolutely GREAT idea you have, and I want to see it done.