It is made out of 1.1 ounce silnylon, noseeum netting, and gossgrain webbing. All the fabric is included with the kit, including the patterns and everything you need, except the obvious sewing tools. They give a list of the things they kit does not include (scissors, sewing machine, pins, sharpie and chalk to mark with, etc.), but the one thing they do not include that they dont tell you they dont include, is the grommet setter.

The side lifters dont do a whole lot, but they do raise the sides a good bit to make a little more room for when there are two people in the tent. They do more than it looks like they do in the pictures. They would also help prevent swag from moisture. The lifters done this way are from the first generation of the tent. The second generation (the one they are currently selling) only has one lifter per side, and it just ties out on a line, with out the extra poles. I like this design better so I contacted quest and got the info for this design of lifters instead of the single lifter per side.

The sliding issue is just the material, no matter what the design this material is SLICK. I did see a tent being discussed on here somewhere with a design similar, where the floor had stakes directly attached to the corners that I assume are to prevent such a problem. I dont expect it to be a problem with the new silicone on the outside, but I'll be sure to post on here the results after I test it again.

As far as sealing the seams, that is explained in the instructions. It is pretty simple though. A tube of 100% silicone (like you use around a bathtub or whatever), and some paint thinner (I used mineral spirits). Both of those are available at Lowes, Walmart, etc. You mix a little silicone in the paint thinner, then brush it on. That same mix is also what you use for the stripes inside and out on the bottom to keep the floor from sliding, and your sleeping pads from sliding on the inside. The instructions include a list of the trails the tent was used on during the design and testing process, and the pct is listed. Since you saying you're from the pacific northwest, I thought that might be of interest to you.

This is the two person tent, not sure if I ever stated that in the post so far, so just thought I'd point that out. As far as the size inside its pretty long, so I dont see being tall being a problem. They say 59in by 92in usable floor size in the instructions. Overall size is 121in long, 108in wide at the door end, and 72in wide at the foot end. The standard height is 39in at the door and 29in at the foot. To muddy all that up a little, the height and floor size vary a bit based of how high you set it up. If you check their site (I'll include these same pics soon) you will see that it can be set up higher for better air flow, or lower for better blowing rain protection. The higher you set it up, the less floor space you have, the lower you set it up the more floor space you have.

Adding to it would not be very easy to do. The floor is based off using the FULL width of the material. Because of that, and the overall design adding to it would be very tricky, and require a LOT of math, and I think quite a lot more sewing experience than just the tent as it is designed.

The answer to the previous question sorta leads into the answer for your last question.... I wouldn't say its really hard, more just very time consuming, but I would definatly not attempt it for your first sewing project. There is alot of talk about the slick silinylon material being super hard to work with, but honestly I didn't find it to be THAT much harder to work with than regular material. I've heard having a good machine makes alot of difference in that, and ours is pretty new so I'd say that helped. The instructions are very clear, so with a little basic sewing experience, its not too bad. The trickiest part of using this material, is that I had to be sure to keep the speed down. Sewing full speed made the material not feed properly and made it bunch up under the foot. As long as I sewed slowly, I didn't have any trouble. Most importantly, make sure you read through the directions before you start and take your time to be sure you understand what you're doing. I think the hardest part is keeping everything square as you cut the parts out. I HIGHLY recomend starting sewing with the stuff sack like they suggest. It gives you a feel for working with the material, on something that it doesn't hurt to mess up (you have enough extra material to replace it if you need to). Also be very sure you follow their layout on cutting out the pieces, because while there's enough extra for a redo on the stuff sack, there's not a LOT of extra included (atleast not with my kit). Long story short, work slow, and follow the directions carefully and its not that hard if you're a little familar with sewing in general in my opinion.

If you have any other questions feel free to ask.