Melt the chocolate chips with the coconut oil in a bowl set over a pan of simmering water. (Alternatively, you can do this in 30-second intervals in the microwave, stirring at each interval.)
To assemble the spiders: Using the tip of a paring knife, make 3 holes on each side of the truffles, big enough to inset a pretzel into. Break off the curved portions of the pretzels (so you have shapes that look like the letter "J"). You should have 6 J-shaped pretzel pieces per truffle; these will be the legs. (The legs do not have to perfectly even, but try to get them close to the same size.)
Line a plate with parchment paper. Dip the curved end of a pretzel piece into the melted chocolate and then insert into a hole in a truffle with the curve going down towards the bottom of the truffle; repeat with 5 more pretzel pieces and the melted chocolate. Lay the spider on the prepared plate with the legs up. Repeat with the remaining truffles, melted chocolate and pretzels. Refrigerate or freeze until set, about 10 minutes.
Flip the spiders over so they are standing on their pretzel legs. Using the melted chocolate, make 2 dots on each truffle for eyes. Stick the desired candies on the melted chocolate. Serve the spiders arranged on a bed of cotton candy.