Excellent duo of customs! Welcome to the Realm!

Firstly, I had sanded the fin off my Red Guardian to make a Black Bolt, but that fig got announced so I went with Bel Dann here- the normal Red Guardian head already has a fin (though maybe smaller?) so you might not have to bother with making a fin at all.
These fins are just cheap sheet styrene- I used a "Beware of Dog" sign from a hardware store, cut a small triangle with an x-acto and sanded to fit the head by glueing some sandpaper onto a sharpie so it would sand in a circle.
I x-acto-ed a slight groove in line with the nose so the fin would have something to stick into, then I just super glued it on his head.
Use just a drop of glue, on the fin part only- then, after it's attached- when you paint the head, the acrylic will strengthen the join.
These guys end up on the floor all the time (cat) and the fins stay on.
I'd love to see you guys make your own versions- they're really fun to customize!

And how did you apply the fins to the head? What material is it? I really want to make these myself.

Thanks guys- a great bunch of replies to my first posting here!
The Folk Art metallics are straight from the tube- they are self-leveling so don't leave brush strokes but take a couple of coats to get to this level of shine. Ony a couple of bucks per 2oz. tube though, so it's an even better deal!
The figs were sanded a bit at the joints and primered with Colorbond spraypaint- it's an autobody paint made to bond to vinyl and plastics and NOTHING is better to primer figs with- you can even use enamels over it and they'll dry (as I did on details on a Black Knight custom which I'll post once I take decent pics). Stuff puts even Krylon Fusion to shame.
The eyes were easier than I'd have thought- following tutorials on painting 35mm miniatures, I painted the eyes black and their surrounding areas dark fleshtone, then went in and added white- took a couple of trys, but is a nice look, I think.
Bel Dan (blue guy) sports a big facial scar from his epic duel with top Skrull soldier Raskorr (FF annual 18).

If you look closely at their weapons, you'll see modified lightsabers- 'cause what's the point in being a super spaceman without a laser sword, right?
I've got the other two branches of Kree military almost finished- purple (medical) and green (exploration).
Anyway, thanks for the comments!

Both look great.Nice paint apps.

Neat, I always thought thst red guardian fig was a good base for a kree warior, but you mae them look better than I thought they could! awesome paint work!!

Regular acrylics? Wow, these look amazing. Did you water down the paint to get it so smooth? Is there any paint rub? How did you get the fins to stay on? How did you paint the eyes? Man, I have so many questions. This right here is quality work.

They look great, like right out of the box!