As long as your painting the canopy, just buy a small tube of acrylic painters caulk. It comes in colors & accepts paint well. Since you won't be putting it INTO your tank, you shouldn't have any problems.
Eheim Pimpette #270
Eheim Pro II 2028; Classics Series -2215, 2217, 2260, 2262; Pro 2229 Wet/Dry