J Mag, I don't have a digitized pattern, but it is really not at all difficult to make something for yourself. You need to start with the dimensions of your Silnylon. I bought mine on Ebay, from this guy: http://www.ebay.com/usr/montanasoftware. He sells by the foot and cuts it from a roll that is 65" wide, which is the widest I could find during my research. Depending on how large you want your Pentagon tarp, you will be visualizing five equilateral triangles out of a rectangle of sil. I wanted the cheapest I could make material wise, while still providing the benefits of a trailstar style tarp. I decided I didn't need the full 7' base dimension of the original trailstar, and opted to do 6.5' to save a bit of money and weight. I bought 20' for around $42.00. 20' divided by 3 = 6 1/2 feet with a tiny extra for incorporating into a seam allowance. You will have two partial triangles left when you are done cutting out your five triangles, which you will use to make laminated corner and edge tie outs, peak reinforcements, a stuff sack for your tarp, and in my case rain mitts to go with my rain poncho, and still have a bit left over for other MYOG ideas. I already had 3/4" grosgrain for midpoint interior loops and peak loops (one outside and one inside the peak reinforcements), and the 1/2" nylon webbing I used for the tie outs (five for the corners, and five for the midpoints along the base). I also had mitten hooks and line locks left from other projects, but you will need five mitten hooks and ten line locks to make my version. You can omit line locks for weight savings of course, but I like to be able to adjust tension from inside the tarp during rainy weather, rather than climbing out and adjusting tension in the rain. I used Gutterman Mara ($3.65 from DIY Gear Supply) for the piece sewing, and Tera for the tie out loops. In all, with shipping, I spent around $50.00 for this project, but to buy everything you need I estimate less than $80.00. You can find the silicone adhesive at Home depot, I used the stuff from liquid nails.
To make the reinforcement patches, I traced the pentagon shape of the peak, made a paper pattern, then cut two pentagon patches from left over sil. If you want it extra strong, you can cut four, and composite two together with the adhesive so you have a double thick patch on the outside, and a double thick patch on the inside. I didn't go that route, I just cut two and used one layer for top and bottom. I used the adhesive to tack on the two patches before sewing them on 24 hrs later. I spread a thin coat on the tarp where the patch would sit, and also on the patch itself, then carefully applied the patches to the tarp, making sure to squeeze out any air bubbles with a wallpaper roller. I used the same method on all the tie out reinforcement areas as well, and there are links on BPL fully describing the process: http://www.backpackinglight.com/cgi-bin/backpackinglight/myog-laminating-silicone-impregnated-fabrics.html#.Uy7xMSij7fs. Makes very secure reinforcements. Make sure that if you want tie loops on the top and under side of your peak for hanging the tarp from above, and a hang point inside, that you apply them to the two reinforcement patches before attaching them to your tarp. It can be very tricky to do a full adhesive lamination on the peak due to the geometry of the tarp when it is not under tension. You have the option of just tacking down 1.5" -2" of the patch border to the tarp, or you can carefully set the tarp up outside, and apply the top reinforcement under tension crossing your fingers that no seams rip at the peak while doing so. I wasn't concerned with perfection, and did it inside, using a 2" line of adhesive along the borders of tarp and patch. It worked well, and when it is done and set up, it looks just fine. Time will tell if it effects the strength, but I double top stitched around the reinforcements through the tarp peak, and I think it will do a fine job of handling the tension. I considered doing the peaks with a heavier material like dyneema grid stop, but opted for the lighter sil.