1) insert pvc pipe (filled with concrete), usually 30" tall because any shorter and people will trip and injure themselves. Not aesthetically the best, and would be in the way on Chili hole 8.
2) Insert metal pole the same height as a basket. (with lock) That way the alt pin can be played as a tone pole if desired. Again, would not work for chili hole 8.
3) Just have two baskets....expensive and again, not for chili hole 8.
4) I'm envisioning a pvc or metal cap that could insert into the sleeve, and have a tab for a lock. This is a homemade one I found on dgcr, before the lock hole was drilled.
5) IMO the best option, though it would require digging up and re-pouring the sleeves. When installing the sleeves, use a Sprinkler valve cover. It is set right into the concrete, such that the cover is just below ground level. No trip hazard, mower will go right over it. You leave enough room for the lock under the lid, and they have holes to let water drain out. Innova even includes this in their installation instructions for their baskets.