Add tab access to check boxes in a protected Excel sheet form.

[Suebriquet]

I want to add a few check box form fields to a protected worksheet set up to
allow data entry in unlocked cells. Tabbing through the sheet skips these
boxes although they can be checked using the mouse.

What do I do to have the user access these boxes via the regular Tab key
progression through the sheet and allow them to be checked with the space bar?