I have put silnylon floors in several tents, but not pyramids.
Spent some time outlining steps for you to install a silnylon floor, and then realized that it would most likely be more work than you would like. Golite uses a heavier taffeta for their "nest" floor, it being the conventional wisdom that it is much more durable than silnylon; but I have had good experiences with floors made of quality silnylon. I think it is because the elasticity of the silnylon makes it hard to puncture or abrade.
In your shoes, I would purchase a similar sized pyramid tent with good top venting and a silnylon floor from one of the "cottage" manufacturers, save yourself a lot of work and frustration, and benefit from a tentmaker's experience, skills and choice of materials, which will be well worth the price of the tent.
Otherwise, the simplest approach that I have used for a rectangular shaped shelter is to make a rectangular floor slightly wider and shorter than the tent footprint; but this may not work with the SL2, as the sides are not straight. But if you want to spend the time, use cheap polyethylene sheeting to get just the right shape to attach to the canopy walls, and transfer it to silnylon. Then install very light snaps along the side hems of the floor and canopy. The light snaps can be purchased at most sewing shops, maybe even Walmart, and come with a little hammer and anvil arrangement to set them into place. I would put them in the hems on the floor and canopy edges, where there are several layers of fabric for strength. This will provide protection, and a slight bathtub effect on the longer walls of the tent, but not at the ends. Suggest you try this first, and see how it works condensation-wise. Sewing up a snap-in tarp is way easier than a full floor, and if you don't like the results, you can just throw it out and remove the snaps from the tent canopy bottom hems, and repair any damage to the canopy hem with SilNet.
I used to use my snap-in tarp as a floor cover in shelters, also. Good luck with the dog. One of my shelties was even able to open the door zippers with her teeth when she wanted to relieve herself. For a younger and wilder rescue sheltie, I just kept her on a flex-leash inside the tent with the handle binered to a stake loop.