I want to make sure that what ever set of casual wear I draw Spike in, it would be a set of casual wear he would prefer to wear often. Some said he would choose a zipless jumper.
Bigdad said he would wear a short sleeve t-shirt and sleeveles vest, And Jeans.
And Flowerpower138 said he would wear a long sleeve t-shirt and sleeveles vest, And shorts.
I even found images of Spike as both a male and female human along with the casual wear that annasabi101 suggested Spike would choose.
1 more thing. PegaSister4life said that when Spike is in his teen years, he would wear a blue and green sleeveless vest with his jeans and boots. The arm belt, normal belt and black boots all giving off a rocker vibe. As did other links I already showed you.