Hi Alex,
Resin will not stick to masking tape. I would suggest taping the bottom closed with a couple layers of tape. Depending on the size of your hole, I might suggest only pouring a 1/4 to 1/2 inch of resin as a first layer. I am concerned that pouring too much at once might be too heavy for the tape to hold. Once you have the first layer in there, it should stick to the sides of the wooden hole and help support future layers.
You should also know that the resin will pick up the sandpaper look of the masking tape. You can either sand the cured resin down with wet/dry sandpaper (start with 400 grit and finish with 1000 or higher) or apply another thin layer of resin to get rid of the frosted look the tape will leave.
Good luck!