The title almost says it all. Can a Boggle use its Dimensional Rift ability in the mouth of a Bag of Holding (or any sack really), or would a bag not provide a solid enough mouth for them to create their rift?
And if they were able to do so with a bag of holding (example, they snatched one and were running off, the PCs were chasing it and it decided to open the bag dimensional rift to hop in and escape), would that be like putting a portable hole in a bag of holding and open a rift to the astral plane?
Including the Dimensional Rift text from Volo's P. 128:
Dimensional Rift. As a bonus action, the boggle can create an invisible and immobile rift within an opening or frame it can see within 5 feet of it, provided that the space is no bigger than 10 feet on any side. The dimensional rift bridges the distance between that space and any point within 30 feet of it that the boggle can see or specify by distance and direction (such as "30 feet straight up"). While next to the rift, the boggle can see through it and is considered to be next to the destination as well, and anything the boggle puts through the rift (including a portion of its body) emerges at the destination. Only the boggle can use the rift, and it lasts until the end of the boggle's next turn.
And clarifying from the Twisting Space section of the text. This is what makes me wonder if a sack or bag of holding would even be capable of sustaining the rift:
Twisting Space. A boggle can create magical openings to travel short distances or to pilfer items that would otherwise be beyond its reach. To create such a rift in space, a boggle must be adjacent to a space defined by a frame, such as an open window or a doorway, a gap between the bars of a cage, or the opening between the feet or a bed and the floor. The rift is invisible and disappears after a few seconds - enough time for the boggle to step, reach, or attack through it.
It talks about frames, but says nothing specifically about the mouth of a bag. And the examples listed in Volo's all mention more solid frames. I did find this on a site discussing tactics the Boggle uses:
The catch is that there has to be some kind of “frame” to create the rift in: a window or door, a hole in a wall, a fireplace, an oven door, a ladder, the end of a sawhorse, the underside of a raised bed, the mouth of a basket or amphora, a hollow in a tree, even the body and legs of a Medium or Large animal. Anything will do as long as there’s a solid perimeter around the rift space, no more than 10 feet on a side.