Combo stock trailer with air gaps, wood floor, mats. No bedding, fly masks on the (barefoot) horses. Mat texture is not slippery.
Previously had a different combo stock that I added solar screens to the air gaps all around – made the trailer noticeably cooler, but hay from horses’ hay bags could get caught between the permanently attached screens and the trailer.
Previously had a trailer with screens and drop-downs (and safety bars to keep heads inside), hauled with drop-downs open, screens and bars in place. In our climate, ventilation is super important, protection from cold pretty much a non-issue.
Have had a few other trailers. Had some extra cushioning added under the mats on an earlier trailer, used to be available online, called something like Safe Step; haven’t seen it lately. No longer haul cross country – just mainly locally, most often an hour or less. Tried bedding back in the past, but not worth the hassle and mess and blowing around, IME. Trailer suspensions are better than they were long ago, too.