Disclaimer: Although I came up with this independently, a due diligence check quickly revealed that someone beat me to it a while ago (probably a lot of people). I am posting the idea here with quick instructions and key words to make it easier to find for people in general but I claim no credit. It does not specifically mention my blanket but the concept is the same. I will show you what I did and you can adapt it to your situation.
The first site I found is here:
Thermarest blankets can be a great way to shave some weight off your pack. The large tech blanket I have is reasonably inexpensive and more multipurpose than a summer weight sleeping bag. This hack should be able to be used for any of their blankets like the Vela or Regulus blankets as well if drafts are an issue. I believe those come with some attachment system but I think it is similar to the sheet. Also, this hack does not rely on the Thermarest fitted sheet so it works for larger ground pads, if you made your own sheet, or don't want to use a sheet at all.
The tech blanket is great for summer camping and into the spring and fall. When it starts to get a little chiller outside, in the spring and fall, the blanket can still warm enough but the drafts from the sides can be bad. Thermarest has tried and somewhat succeeded in keeping the drafts down if the blanket is used in conjunction with their fitted sheet. This is still a bit drafty but much, much better. This hack takes that idea one step further to seal out more drafts using just a few feet of cord. Really, you are turning the blanket into a light weight sleeping bag when used with a ground pad.
There are many variations on the idea either using para cord or shock cord and the never ending ways to lace the existing tabs into something that suits your fancy. I would recommend trying a couple variations and see which works best for you in different temperatures. Another method would be to sew a cord sleeve all the way around but I find this to be too much work for not enough benefit. I am 5'10" or so and the large blanket works for me but it is pushing it. Someone larger might not be able to make this work for them.
Step 1: Set Up
Lay out your tech blanket on the ground facing down. Inflate your ground pad and center it long ways on the blanket.
Step 2: Lacing
Take a good length of cord and lace it the way you would like. My personal favorite so far is just a cross lace like on a shoe. This has been the most effective for me so far since it does not have the possibility of popping up off the pad if I turn over at night. Also, for me, the foot box works better if it is on top of the pad so I make sure to pull the existing cord on the blanket to make the foot box and then place it where I would like it and lace it through. Not lacing it through at all will allow you to kick off the foot box all together if it gets warm which can be fantastic for heat regulation. When lacing, do not actually put the cord through the bottom foot box cord by accident.
Step 3: Measuring and Cutting
Once you have it laced, tighten it till the edges of the blanket are near or touching the edge of the ground pad. Make sure to leave a few inches as well on the ends to be able to grab and make a knot later. Cut the cord to length.
Step 4: Finishing the Ends
Once you have the cord cut to length, the options for finishing it are also numerous. If you want the 550 cord to lay flatter or you want to save some more weight just take the insides out. Once you have things the way you want them, finish the ends of the cord you are using with the appropriate means so it doesn't unravel. For my 550 cord, that means taking a lighter to it to seat the ends. For shock cord, you may need to do something different depending on the type you got.
Step 5: Finishing Touches
After the ends are finished, I added a little friction lock I got from an old draw string bag to mine to make it easily adjustable. To do this, thread the two ends and thread them the same direction through the hole before putting an overhand knot in it. The friction lock is not necessary; you could just tie it but, I find that it makes it easier.
Step 6: Use!
Since the tabs snap together it is easy to take this on and off. Your cord just ends up being a giant loop. I store it in the little pocket when I am not using it or just stuff it in the pocket with the blanket when it is folded in on itself.
The way I set it up is to start at the bottom and cross the cord over every time I move up a tab making sure the friction lock is on the top tab. After that, I put my ground pad in over the cords. When I am ready for bed, I slip in and pull the cord tight enough for my own comfort and enough to seal out drafts. This may mean that part of the blanket goes under the pad which is part of the reason this works so well. ONE WARNING! If you have a delicate ground pad or air mattress be careful when tightening the cord so as not to pinch it or adjust it so quick it makes a friction burn hole in the pad/mattress. This is unlikely but there are those people out there... they are the reason we have warnings on cups about our coffee being hot...
Hope this helps make you next trip to the back woods a little more comfortable!