I figured as such… so you do have a few options as mentioned above.
here are a couple more I would also suggest to add to their options
Use TagWriter or NFC Tools and write out your keys as a plain text NDEF, depending on how many wallets you have your total size should be less than ~888 bytes
You could buy a dedicated xNT for example and write that record to that implant, and only use it for that one purpose.
Pretty cheap and safe way to store the info.
I would also mention, you would probably want to Read it at least once every 10 years, but more frequently would be better, just to refresh the data retention. (another physical backup would be sensible)
Don’t tell anybody you have it, put it somewhere less convenient than a hand (Thigh, back, shoulder, bicep…)
put the location and information on how to access it and what to do with it in your will. (makesure your will is enacted before you are cooked 
or buried.)
The chances of somebody scanning your body whilst you are alive and stealing your keys, in my opinion are very Low.
Unless you are super loaded 
but then still do the same, but also
Another option is use something like OpenKeyChain : Easy PGP to encrypt your data, and write this to your implant and again to decrypt when you read it
Another is get a NExT now, because they are awesome and wait for the SOON™ to be released Apex that which have a number of options for you including encryption and key storage
