Never tried to make a "trailer" blind. I love this sort of stuff !!
I have made many, many box blinds. Most elevated. Here's some thoughts. Use pressure treated materials for the frame and floor for sure. If you are planning on having three adults in the stand at one time (plus gear + heater, etc.) it needs to be about 8 x 4 (8x5 is better). Otherwise it'll be mighty cramped. 6'6" inside height should be tall enough. And easier to heat than an 8 ft. tall box.
If you can afford pressure treated plywood for the outside do so. If not try 4-ply decking plywood for the exterior skin, well painted. That is what I use and this lasts for years if painted regularly. I paint my blinds in a cammo sort of pattern. I also stain the inside with a dark blackish wood stain to help cut the profile of the hunters.
Because it sounds to me like you'll be moving it about a bunch, be sure not to scrimp on putting the frame together. If you do, the frame will likely loosen up. I strongly suggest that you use quality screws to put the stand together, rather than wire nails to assemble the frame and attach the outer panels. Nails will flex better but I think the screws will result in a stiffer, quieter stand.
The stand will need to be well bolted to the trailer bed for sure.
As for a roof, if you are going to be towing at highway speeds, I'd recommend a well attached plywood roof, covered with tin. Would be strong and 100% rain proof for years. Might consider putting carpet in the floor to help mitigate moving about sounds. As far as the sliding windows, I have no clue. I don't build mine that way.
Post a photo of the end product. That'd be cool to see.