It was often the case that wooden trestles used what were called sills to support the bents off the ground and away from water. There were mud sills and timber sills. What you ought to do, if you'd rather keep the timbers free from plaster, is do a series of fittings, marking out where each bent will need a timber sill. Scour out a shallow channel where you make each mark, and actually stain and set a suitably cut timber in that channel. Make sure it is level and square so that the bottom timbers of each bent are not going to meet a canted or twisted timber.
This will take some patience....so pace yourself. Get it right. Test fit, scrub, shift, cut, gouge..keep at it until all your timbers support each bent. Don't be afraid to trim the bottoms of bents if it will save you some aggravation. Just don't trim them too long...if you get my drift. Patience. Give the structure all the provisions for remaining faithful in its service to you when it is meant to support your trains.
Once the timbers are situated correctly, you can caulk them into place or pour a thin layer of plaster around them, or wedge sculptamold around them. Let them set into place, and then make abutments higher up at the ends where the bridge deck is to end. If you get the sills right, you won't even have to glue them...just glue the stringers and decking that runs for a short distance over the tops of the abutments. That is all I used because I made darned sure the bents at their footings were sufficiently firm and solid to support the bridge properly all by themselves....without any adhesives!
