TBHers might have an input here. 3x25mm holes enough?
22mm by 420mm is far more than enough for any colony. I often simply used the floor with the small lip upper-most; Dartingtons have an 8mm slot and it might, or might not, be full width all year round, (unless potential robbing is an issue).
Look at the recommendations for bait hives - 25 x 50mm? That would be what the bees would choose? No rocket science, just a few observations and a little common sense is all that is needed.
I have likely lost most of the original entrance blocks and simply use lengths of 20mm-ish planed-to-fit square strips, or perhaps a narrow frame foundation-clamping bar, or whatever might be at hand and do the job. I usually have a length of planed strip in the garage and cut off an appropriate length as required.
So the only issue might be if it is just too small for the size of the colony or too big to protect from robbers. Winter is not an issue (especially with OMFs!) except for keeping out the mice, etc.
Regards, RAB