Florida statutes provide for appointment of a health care surrogate, which may be of value in preventing autopsy: Fla. Stat. § 765.202 Designation of a health care surrogate
Fla. Stat. § 765.203 is an example form which may be used to appoint a health care surrogate.
Note that this form is a boiler-plate form which makes no mention of cryonics.
Before creating a cryonics specific form, consult: Medical Power of Attorney for Cryonics
(1) A written document designating a surrogate to make health care decisions for a
principal shall be signed by the principal in the presence of two subscribing adult
witnesses. A principal unable to sign the instrument may, in the presence of
witnesses, direct that another person sign the principal's name as required herein.
An exact copy of the instrument shall be provided to the surrogate.