There are several solutions I am considering:
1) cutting the cakes in half. I'm not sure whether to bisect them laterally or horizontally. A half-cylinder lying flat like a little grow log seems neat, but a half-cake in a trimmed cup might be better on space.
2) mixing vermiculite and watercrystals to create a more stable surface.
3) sterilizing some toothpicks and using them to prop up the cakes. I like this idea the least.
1) No need to cut 'em up. Too risky and damages the cake, requiring spending energy to recover from. Crumbling them to spawn to grain or bulk is a different story, however. Try to tear mycelium instead of cut it whenever you can (cutting breaks cell walls, tearing keeps cells intact).
2) That won't provide as much of an increase in stability as you would probably prefer and is messy as hell.
3) Stay away from toothpicks or any wood. They don't stay sterile long.
Something tall and skinny like the jars you're using get a lot more stable if you lay them on their side. Place them right next to each other and you can add the gel to the space formed between them (if two cakes on their side look like OO as seen edge-on, put some gel where the quotation mark is: O"O plus fill the lower space with gel/verm mix as a second reservoir). Don't use too much gel on top since they don't like fruiting right through the gel itself.
Poking into a cake w/ a dowel or something and filling the space w/ saturated gel also works (a variation on a rez-effect technique except without crumbling the cake- lots of good threads in the rez-effect vaults). The poking is best done at the beginning when you're making up the jars otherwise you risk busting them in half, which is not a big deal but can be annoying.
My thinking right now is that top-dressing with the gel is less effective than mixing it into the substrate in terms of benefit; gel set on top of cakes has sat there shrinking very slowly while gel I mixed into a casing layer vanished fairly quickly, indicating the water had been wicked out if it. This stuff is only effective if we can get the water back out of it, and the more surface area of substrate or casing layer it contacts, the better.