What could i use to heat / reheat food for an outdoor wedding reception?

My fiance and I hired a single caterer to do the food for out wedding coming up in October. He did not want a lot at all for his services and he has a lot of skill. Part of the problem I am having is he does not come with hardly any equipment so any electrical, grills, hotboxes, etc would be on us to rent / buy. We will have at least a couple Cambro hotboxes for the event but being on a private patio and completely outdoors means we do not have access to a kitchen. So many things have to be cooked and transported and held on site until serving. Many of the items can be heated before hand however, a few of the items could really bennifit from having some kind of oven to heat things up on site. We have bought portable burners and even a flat top grill for one of the main menu items. Getting to the main point, I want to see what anyone has used or seen that can heat food with possibly standard electricity or propane gas that can act as an oven without spending $400 to rent a large scale oven.