I didn't read the spell carefully enough to notice that the inscribed object isn't protected from the explosion. Perhaps a custom version one level higher that doesn't harm what it's on?
Instant Summons doesn't work very well when "the item is in the possession of another creature". Sure, it tells you where it is, but you still have to go get it and the thief could move before you get there.
The Sigil doesn't trigger until someone tries to read it, which would probably be either never or when the Sorcerer inspects it, and he'd probably take precautions against such things.
Perhaps one of the Symbol spells, set to trigger when an evil creature touches, or maybe looks at, the book?
You could also stow it in a
Secret Chest, though you'd have to recast the spell every time you put it away.
Unfortunately, all of the options I've looked at besides explosive runes have expensive material components or foci, ranging from 500 gp for Sepia Snake Sigil to 5050 gp for Secret Chest. If we're willing to spend that much, we could try adding
Forbiddance to the existing Permanent Mage's Private Sanctum, but that could get a bit harsh on random curious bystanders who want to investigate the mysterious fog bank or just happen to be too close when we drive the wagon by. Hmm, it's shapeable and I think the volume given is a maximum, not a fixed value; is the wagon at least 10 feet in every dimension?