$fn = 360;

h= 30;
t = 30;
r = 100;

module tube(h,r,t) { //t for thickness
    difference(){
        cylinder(h,r+t,r+t,center = true);
        cylinder(h+2,r,r, center = true);
    }
}

// bolt opening

module boltnut(){
    br = 1; // bolt radius
    nh = 1; // nut height
    nr = 2; // nut radius
    cylinder(h+1,br,br,center = true);
    translate([0,0,h/2-nh+1])
        cylinder(nh+1,nr,nr,center = true);
    z = h/2-nh+1;
    translate([0,0,z])
        cylinder(nh+1,nr,nr,center = true);
    translate([0,0,-z])
        cylinder(nh+1,nr,nr,center = true);
}

difference(){
    rotate([90,0,0])
        difference(){
            tube(h,r,t);
            h2 = 10;
            t2 = 10;
            tube(h2,r+t-t2,t2+1);
        }
    // remove bottom    
    s=r+t;
    translate([-s,-s,-2*s])
        cube(2*(r+t));    

    x = r+t/2;     
    num_bolts = 10;
    for (i = [1 : num_bolts ]) {
        rotate([90,i*180/(num_bolts+1),0])
        translate([-x,0,0])
        boltnut();        
    }
}

bh=20; // box height
x = r+t/2;
translate([x,0,-bh/2])
    cube([40,60,bh],center=true);
translate([-x,0,-bh/2])
    cube([40,60,bh],center=true);