I just stumbled over an old pack of "Plasto", a plastic filler from Revell, which I used for plastic model kits. It turns out that this is a pretty good filler for keycaps, too! It cures without external heat and results in a durable and stable inlay. This stuff is also a good alternative if you do not trust the cheap modeling clay I tried for my first batches (though the clay itself withstands a good amount of screwdriver scratching when in place and dried).

Other really stable alternatives would be two component fillers (like green stuff) or heat-activated plastic materials (like Fimo). The correct coloring is always a problem - not all materials are available in all color choices. But there are definitely a lot of good alternatives out there to create really stable keycaps using the inlay method described on this page. Just be creative! :)