I agree on ventilation, but insulation is insulation. The more the better. The more insulated the fridge is the better it will maintain temperature at lower energy consumption, enclosed, in the open, sunlight, or shade. The percieved difference may be more in the worst case senario, but insulation is benefiting the fridge at all times.
It is important to get hotter than ambient air away from the fridge, but at some point, ventilation becomes an enemy. Once you've removed any heated air your ventilation is then fighting your insulation by bringing in a constant supply of fresh ambient temp air to try to dump it's heat on your fridge that hasn't already done so by being next to it.
Hot air rises, so it would probably be better all around to make sure it has ample room to escape on it's own with clearance than to actively push air over the whole fridge and accelerate heat transfer as a whole. The internal fan in the fridge moves enough air to get it out of the inner workings. A secondary fan might be an option in an extreme case, but at that point you probably painted yourself into a corner with poor fridge placement.
Bottom line, insulation on the body of the fridge to supplement it's internal insulation is never a bad idea or not helping. Actualy in the case of the too enclosed and trapping it's own heat issue, it's going to help more because it may offset the loss of effeciency by having to move more air around it to shed heat.
In term of efficiency, there is no downside to properly done insulation, but the effectiveness diminishes very rapidly. If you look at insulation vs thickness graph, more insulation is improvement, it's just a question of how much improvement are you getting vs. the cost and trouble of insulating. It's a cost vs. benefit ratio. This is a generic graph, but the overall shape of it should apply.
If it were so easy to improve the efficiency of these fridges just by adding a another layer of insulation, why wouldn't the manufacturers do it in the first place? Spray foam Insulation is essentially free, and of course efficiency is one of the biggest selling points for these portable fridges. If the manufacturers thought they could noticeably reduce their power draws just by adding another half inch or so of insulation, it would've come like that from the factory. But the fact is. if you look, almost all the fridges and coolers from every manufacturer are about the same thickness and roughly have the same amount of insulation, so there must be some break even point in which added insulation really isn't providing much benefit.
For added ventilation, I don't think there's a downside if done properly, except for cost and complexity. The point of extra ventilation isn't to flow more air around the whole fridge, but just the working part that's generating heat. The compressor of the fridge will always be hotter than the ambient temps no matter what, so improving airflow through that area will always be a benefit. There's also a point of diminishing returns for fans too, but for a fridge that's used in enclosed spaces, like most area, I think this might be a better way of improving performance than added insulation.
I haven't had a chance to take detailed measurement of my testing, but I'm guessing the reduction in surface temps will more than offset the additional 0.15W of power draw the added fan is using.
I do think a cover makes sense if the fridge sees any sunlight at all. The exterior of the metal fridges get really hot the sun, so any type of cover that provides shading would make a huge improvement.
