MollyJanet I expect filters to increase back focal distance by a little, but I am surprised it is so much that a filter holder sold for use with Canon cameras cannot actually cope with filters.
Not your fault, but ZWO's. None of their diagram with filter drawers and filter wheels mentions that you need to compensate for it.
Measure the thickness of your glass.
Take about 1/3 of that thickness (assuming glass has refraction index that is close to 1.5) and that is the distance you need to add to the drawer (i.e., 0.5 out of 1.5)..
Good 2" filters (from IDAS, Radian, for example) are typically 3mm thick, while cheaper 2" filters are only 2mm thick. In any case, you will need at least 0.66mm worth of spacing to achieve infinity focus.
ZWO includes some plastic spacers with some of their products, but to get 0.6 to 1mm worth of spacing, they are pretty flimsy.
Places like Agena sells metal (brass or aluminum) spacer washers (usually 0.25mm, 0.5mm and 1mm thick) that are much better.