I have the following template:
{{session \"modalTestNumber\"}}
Using the latest version of Meteor, here is a simple way to render a doc into a bootbox
let box = bootbox.dialog({title:'',message:''});
box.find('.bootbox-body').remove();
Blaze.renderWithData(template,MyCollection.findOne({_id}),box.find(".modal-body")[0]);
If you want the dialog to be reactive use
let box = bootbox.dialog({title:'',message:''});
box.find('.bootbox-body').remove();
Blaze.renderWithData(template,function() {return MyCollection.findOne({_id})},box.find(".modal-body")[0]);