Do I need a mounting kit to install this board UNDER a wood burner? Id like to sit on the floor bit do I need spacers? Can it just sit on the floor?

Yes. The amount of protection needed under a stove depends on its design. The minimum protection need for any stove is a floor protector extending at least 18” (457 mm) from the loading side and 8” (203mm) on the other three sides. (The area must not be less than that specifed in the heater’s or stove’s installation instructions). This is not intended to provide protection from radiant heat but to prevent live coals from burning the floor. Certifed stoves have floor protection requirements listed on the label