I know this is such a hack, after spending many hours of not getting it, I used it anyway.
<a href="/expenses.xls?<%= request.fullpath.split("?")[1] %>">make xls</a>
so basically it takes the searchpath after the ?, then adds it to your model.xls output path and then it works. I hate it myself, there must be a better way, but deadlines.
There was a good link here.