An IOU?
"Uh, sorry but I didn't have the time and I'm too broke to buy or build you anything. I'll get back to you as soon as I can though." Yeah, that wouldn't be awkward at all.
I think a hundred bucks in cash or a check placed inside of a beautifully worded hand written card is a fine gift to any newly weds. Unless they are well off financially they can use the money for something special and even if they don't really need it it certainly won't get shoved into a closet or returned to a store for a refund like so many wedding gifts. A handmade gift is great but unless you're truly inspired and don't make them wait, give them the cash.
Bruce