Re: sealing up openings
bh, I'm figuring that you're using belly tape or either something like duct tape. If the belly has rips in it and you're trying to hold the edges together and tape at the same time then you have your hands full. You could try using short pieces of tape to hold the edges together and then come back over it with a strip of tape to do the sealing.
Something else... Think about those sterile pads with adhesive around the edges that we put on wounds...kinda like a bandaid. You could make some large pads out of clear plastic sheeting, garbage bags, or whatever (sheeting would be easier to work with). Get the tape around the edges before you get under the trailer...then when you get underneath you can position the "pad" and press the uncovered edge of the tape onto the belly without having to gather the edges of the rip together.
Cardboard wedged in between the stringers that come out from the i-beams could block off the bees from larger areas...staple one edge to the subflooring and tape the other edges.
Insulation could be wedged into cracks and crevices to block passage.
Warning: Rookie beek...take my postings with that grain of salt you keep in your pocket.