Mike, in theory a properly operating diesel engine emits no fumes.
Fumes mean there is a diesel leak, or there is spilled diesel (like from purging the lines) that has seeped into raw wood, etc. under the engine. Or there is something like oil venting out from an open vent gap, bad o-ring on an oil fill, etc.
I'm not sure I'd ever think any engine could be maintained well enough to sleep over it. Any solid barrier (mylar, metal foil, lead soundproofing, roofing sheet metal, roofing elastomer, etc.) should block the diesel but a surgical cleaning and extreme attention to detail are the best way to start. Positive ventilation (a day-night solar vent) may also help.
I think most diesel owners make the mistake of not replacing the crush washers when they crack fuel lines to purge air. From what I've been told, you can get away with that but for a perfect seal, the crush washers really should be REPLACED every time. (Inexpensive by the dozen.)
Any raw surfaces (wood, plywood) should be throughly cleaned, dried, then sealed, preferably with an epoxy paint or sealer. If you can get a small steam cleaner, that's probably the most effective way to get everything off. And then, of course, you'll need to apply something else to any raw metal to prevent rust, because steam-cleaned raw metal likes to rust fast.