>Does anyone use a imirie shim?
I have a few but mostly I use them for extra space for a queen cage on top of the top bars for introducing queens or providing a temporary entrance between some boxes or providing extra space for a pollen patty on top.
> Can you add the shim to your colony at any time?
You can, but during some times under some comditions you'll end up with a mess of burr comb because they violate beespace. If you have empty supers on and they haven't gotten crowded they usually don't seem to burr them much. If you let them run out of room they will put an inch of burr between the boxes on every frame.
Two shingle shims on the top box with the inner cover on that and the cover slid forward works better with more upward ventilation and more of a top entrance. A notch in the inner cover and the top slid forward works pretty well also. But the Imirie shim can provide a middle entrance. The ones from Betterbee are a little more sturdy than the ones from brushy Mt. If you want to make your own, I'd be tempted to make it just 3/8" with metal to reinforce the corners and over the gap for the entrance. That way you'd get less burr than the 3/4" thick ones.