My wedding cake last week has SMBC and the icing was pretty mushy. I have 2 weddings this weekend and I'd like to find a good recipe close to a meringue buttercream that will stand up better to the heat. Any suggestions?

Meringue buttercreams are very heat sensitive. I use the whipped cream buttercream icing in the recipe section. It starts with a royal icing, meringue like base, but is stable with shortening.

Try using ice cold butter it is a bit stiffer. Also chill it well.

I love IMBC but it doesn't hold up well here in the heat and I know how hot and humid Pennsylvania can be too. I just try and refrigerate my cakes overnight and right up till delivery time, keep my car in the shade or the garage if you have one. Then start the car up just before you are ready to leave and turn the air conditioner on.
Some people use blue ice packets around the exterior of the space where the cake will be riding. One woman on another forum uses a tall box and cuts the side to make a door, turns the box upside down so the top is solid. She uses the small aluminum trays from chicken pot pies, fills them with water and freezes them solid. She puts a non skid shelf liner in the bottom of the box, and fills it with the frozen pies, a piece of cardboard cut to size on top of the ice, another rubberized shelf liner on top of that, and then the cake. She tapes the door of the box closed, uses a brick on the ouside of the box on all 4 sides to support it. She says she has traveled 60 miles in summer heat this way and the cake was still cold when they arrived at their destination.

What if you use SMBC under fondant? Will there be any problems due to the heat?

I use it all the time under fondant!

So in heat it will be okay under fondant? Someone wants me to do a cake with smbc underfondant, but I have little experience with it. Would I need to do anything different than using regular buttercream?