I Suppose....

[Donald Draper]

I say, "Check-in is between 3-7 PM, about what time do you think you'll be arriving?"
And I get, "Somewhere in there."
Some days I reply if we're not here we'll leave a note with a phone number you can call when you arrive.
Sort of surprises people that someone might NOT be here. But, we do tend to not make any plans between 3-7 PM..

We say until 5:59PM. Somehow being specific about it makes them realize that at 6PM we are out of here.
Maybe what we need is a form.... Please select you arrival time:
2:59 or earlier - Email us to pay fee
3:00
3:30
4:00
4:30
5:00
5:30
6:00 or later - Email us for instruction
Please note, we reserve the right to not be here if you are more than an hour late. So better email us for instructions if you aren't sure.
Heck, maybe we should just stop doing arrivals and send everyone the self check in.
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We used to do check-in 2-6 pm…by 6 pm we were tired and hungry and in no shape to try to be hospitable. Now we do 2-5, with the expectation that someone will always zoom in at 4:59 but we'll be DONE by 6 pm. That has helped us a lot.
Some days if there's only one check-in and they haven't given a specific time we leave them a note for self check-in. We get to have a life too, especially on a Sunday afternoon. Gotta pick your battles...
.

I'm starting to wonder if maybe we should send everyone the self check-in information ahead of time and say the code works from 3PM. You can ring first, if you want.
Oh and if you want help with parking, restaurants, etc. then please give us an approximate hour of arrival, because otherwise we aren't there and you are on your own.
.

That seems like it would work well for you. We don't have to worry about parking explanations as you do but directions are a big thing so I make sure they have them if I won't be here to help them.
Hang in there!