Hi Lenny,
You sound like you already know what your'e doing; you want a skirt using 1.5 " 'logs'. Easy! ( I did this), decide how large you want your skirt to be, I used the width of our tree as a guide, I wanted the skirt to come to the edge of the widest branch, plus the height of the tree stand (9"); that gave me a 50x50 square ( you might want an octagon, etc..) I decided the blocks would look best with 3 'rounds' ( you know, 3 logs , each side from the center) I just made blocks untill I thought I'd have enough, got extra, made a quilt, but, that's another story.... I thought I'd like the skirt to tie, but you could use buttons, or frogs... measure the circumference of the trunk, that's your center opening size; I quilted the top, folded it in half, then half again to find the center, drew a circle, cut it out, and used a rotory cutter to slit the top. started the binding on the slit, near the center opening, ending at the other side. bound the center with a longer piece of binding to create a tie, and TaDa!!!!! Don't forget to show us your finished skirt! Good Luck!