Generally in the case of noticeable reflections, you should check at first if it is due to light scattered from the edges of the lens. You can try to put a black construction paper with an opening 10 mm less than the diameter of the filter, between the filter and the telescope opening. In photography, this kind of visor can create huge contrast enhancements, especially with camera lenses. In this way (with the additional construction paper in place), you can adapt to a “too large” filter opening without looking hideous and reducing the heat from the telescope. If this doesn’t work, you can try a smaller opening, for example 20 mm smaller than the diameter of the filter.
We hope our suggestions help you achieve the best results in photography. In amateur astronomy there are always some fine tweaking to do by yourself – That’s the beauty of it.