The only research I have kinda done into the SV fashion blocks is on the bag block and I assume all the rest follow the same format which is 8 bytes for an item and it doesn't appear for any set item. So when you purchase an item it just modifies the next unused space. For the 8 bytes, the first two bytes seem to be an items id (from the few examples I have looked at; though all ended in 0B so take this with a grain of salt) and 5th byte is 0x01 if it is new and 0x00 after being viewed.
So for the canvas bag mystery gift, if the mystery gift data manager doesn't work for you, you can export the block, change the item bytes in an open slot from 0xFFFF to 0xE70B (this should be the correct id) using a tool like HxD and importing it back into your game to try to get the item.