Reddit reviews Frost King V76H Shrink Window Kit 84-Inch by 110-Inch, Clear

Swiss Army Classic .74oz

Frogg Toggs jacket 6oz

Polycro ground sheet or trim your own 1.6oz

This will save you over 19oz for about $40

Just get a shrink film window sealing kit with plastic film large enough to cover your opening. You can apply the tape to the metal frame of the upper part, and the top and face of the lower screen and screen flap. The film won't mind that there are a few "steps" into the room involved. You can seal to the painted alcove wall and the sill, but check first in an inconspicuous spot that the tape doesn't damage the paint.

Just made one for a UL2 out of polycro with tyvek corners reinforced with grommets. Here are some links to the stuff I used.

https://www.amazon.com/gp/product/B000AXON8C/ref=oh_aui_detailpage_o06_s00?ie=UTF8&psc=1

https://www.amazon.com/Lord-Hodge-1073A-4-Grommet-Kit/dp/B00004Y692/ref=sr_1_4?ie=UTF8&qid=1485622147&sr=8-4&keywords=grommet+kit

Made my own tabs but they look like this.
https://www.amazon.com/Ultralight-Tyvek-Grommet-Tabs-Campcovers/dp/B00J44KWK6/ref=sr_1_2?ie=UTF8&qid=1485622176&sr=8-2&keywords=tyvek+tabs

I used this kit.

Hardware store is your best bet. If you don't live near one I just got this one on amazon which can be cut into two 7' x 4.5' groundsheets. It's .7mm and there's also reviews of it being used as a groundsheet

The materials I used are

• 84 x 110in shrink wrap(7x9ft)
  
• weatherproof tape, the one linked isn't the stickiest, but does the job
  
• that's it!
  
  You need to tape over all the edges and add tieout points.
  
  For the tieout points, you want to reinforce the area you're gonna attach the tieouts to, then get a 8" length of tape, fold it over on the long side so you get a strip, then one end on each side, tape each side until only ~1" is sticking out. Holds 20 pounds on each tieout fine.
  
  Taping will require an hour or so.