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Sorry, I don't actually understand the problem...
Sealing the rear cell opening should be easily achievable with a large clear filter such as a 72mm or more. This will ensure you don't experience any vignetting using 2" (50mm) accessories. You'd need to obtain the appropriate adapters to make this happen. I'm not sure why you'd want to leave the filter there when using the telescope though. If the filter is simply a clear filter, i.e. not a nebula filter which will provide some visual benefits. It is just another piece of glass along your optical path potentially reducing the amount of light reaching the focal plane. Also the addition of adapters to mount the filter will increase you telescope focal length. This may or may not be what you want to achieve.
Can you provide some reasoning/logic behind why you want to do this? Perhaps just a standard 2" plug is a better option.
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