A tight guard, good blocking and evasive body movement. A good defence is a great foundation for any fighter.
The slower you are the better your anticipatory defense and positioning skills have to be. High guard will limit a slow fighters offense pretty significantly. Golovkin is a good example of a slow fighter. Very slow now but he can anticipate punches and look pretty slick at times with subtle defense. However due to his lack of speed and overall stiffness he still gets caught flush often.
Keep a high and tight guard, constant bobbing and weaving, use the legs to be mobile, or the cross arm defense ala Foreman and Moore. I’m sure there are other things to do but these came to mind right away.
Like other's have said, high guard, head/upper body movement, intelligence and anticipation, and good footwork.
You thinking of taking up boxing, Mark? My advice is just pick an opponent that even slower than you are.
Glencoffe wasn't fast, so he used a hellified offensive work rate as his best defense. Same with Brandon Rios, slow guy, but his over the top workrate against Acosta, Peterson, Antillon & Alvarado made them cave in. That style doesn't last long, but some fighters best d...is their high octane offensive work rate.
An elite impregnable high guard (Think Arthur Abraham, Winky) and developing an ability to catch and counter (Think Stiverne, Curtis Stevens). And yes I know Winky and Curtis aren't slow but their styles allow them to be able to sit idle and not have to use much movement if they choose not to. For a slow guy that likely can't move out of the way of most shots, less movement but being able to protect yourself is of the utmost importance.