What brand and model of coma corrector are you using? Does it have instructions?
Generally, there should be a specification for the coma corrector that tells you how much spacing needs to be between the corrector and the image sensor. Then you need to assemble the various spacers and connectors to meet that specification.
For example, let's say the optimal spacing for your corrector is 85mm. The distance from the front surface of the 294 camera to its image sensor is 6.5mm. Then if you attach the 11mm T2 ring that comes with the camera, you have 17.5mm of total spacing. That leaves 67.5mm of space that you would then need provide via spacers. Or, if your corrector has a spacing requirement of 55mm, then you'd need to provide 37.5mm of spacers.
Then you need to make sure you won't be adding so much length that you can't come to focus anymore. This is a common problem, especially when folks start using a coma corrector, a filter wheel, a DSLR (with their 44mm or more flange to sensor spacing) and a t-ring, etc. The length of everything just gets too long and you have to find a way to make it all fit. (Things like replace the focuser with a lower profile version, or move the coma corrector inside the focuser, for example.)
But the first step is to make sure you know what your coma corrector specifies for proper spacing.
