We do rather well just using 9X12 tarps over the ends of our off-road POPUP. Before raising the roof top I drape the 12-foot end of the tarp thru the roof rain gutter and hold in place with a 52-inch stretch rope. Then I go down from these two hook ends down to the the trailer frame which very securely hold the tarp in place.
Now I can go ahead and raise the roof up. This is great to do even in the rain as it gives you a place under the tarp to work getting the roof up and the bed ends pulled out even during a rain period.
The tarps are then secured in place using the tarp eyelets and smaller stretch ropes securing to the framework under the tent beds making a nice looking folded tarp over the whole bed end like shown here... (LOWES/WALMART)
I basically get the same results as using the GIZMOS but at a much reduced cost. My tarps will last around three years. Doing this keeps us cooler in the summer and warmer in the winter and really stops the condensation problem.
I also use the 2X2X36-inch foam pieces that are used when installing an house window air conditioner unit between the bed end material flaps and floor area which really stops up all of the small draft hole around the bottom of the tent material after you attach it to the under floor catches. I will use several of these foam pieces all the way around the floor area. This really work great (LOWES)
We POPUP owners also have a gap between the wood floor and the front/rear metal popup wall where a small angle rubber seal is used to seal off the junction when the bed ends is pulled out. In my case i can lean over and look straight on at this interface and can see daylight. Bugs and mosquitoes can enter the inside of the trailer here. I add this piece of weather strip cut in a couple of places to allow for the floor roller wheel and slips these in place to completely seal off the air gap. You could make this a permanent thing but I just do it everytime we setup. Also really helps seal out the outside. This basically just slips over the top of the metal wall and being slit like it is stays in place. There is other slit rubber seals that will work as well... (LOWES)
Sample photos from google images
This works great for us keeping our off-road POPUP sealed up pretty good from the elements at a low cost. My bed end tent material looks just like brand new since being used since we purchased the trailer in 2008.