It looks like you will need to do a straight cut from end to end. While a router would probably be best, access to a table saw would work as well (sorry, no clue if you can use a friends, etc.) Just keep the blade the depth of the cut you need instead of all the way trough the board. Cut a first groove and then move the fence just a little, make another cut, etc. You can do this until you have reached the desired width.
Guys with more experience may have some reasons that this is a no-no so check that out first.
A table saw with a dado blade would be easier.
Consider all your options though...for instance if you know you are going to add t-molding you might want to plunk down the few dollars for the router etc.
Good luck.
