If you solved the puzzle using a Vec buffer you can try solving it without the buffer as a stretch goal. You may find the slice methods that let you mutate a Packet's data useful, but remember that the first six bytes of your Packet will be the random device address - you can't decrypt those! A solution that does not use a heapless:Vec buffer can be found in the src/bin/radio-puzzle-solution-2.rs file.
If you solved the puzzle using a heapless::Vec buffer and a heapless::LinearMap and you still need something else to try, you could look at the Vec and BTreeMap types contained within liballoc. This will require you to set up a global memory allocator, like embedded-alloc.