My saving for Europe is shocking at the moment! This phrase may come in handy...
why not add the 'ett' in front of the 'hundra'? ----kan du låna mig ett hundra kronor?
You can do both that and use the compound word etthundra, but we usually don't. hundra on its own means 100 so adding ett is redundant.
As Swedes write the numbers to 36 as words, 100 should be a valid solution here.